diff --git a/qa-tooltips.py b/qa-tooltips.py
new file mode 100644
index 0000000..eb2ceaa
--- /dev/null
+++ b/qa-tooltips.py
@@ -0,0 +1,132 @@
+#!/usr/bin/env python3
+# Headless QA for glossary tooltips (issue #350): parses the built HTML in
+# public/, re-implements the same tag-skip rules the JS uses, and asserts the
+# acceptance criteria. Run from repo root after `zola build`:
+#   python3 qa-tooltips.py
+import json
+import re
+import sys
+from html.parser import HTMLParser
+from pathlib import Path
+
+ROOT = Path(__file__).parent
+GLOSSARY = json.loads((ROOT / "data/glossary.json").read_text())["terms"]
+DEFINED = {t["term"].lower(): t["definition"] for t in GLOSSARY if t.get("definition")}
+
+SKIP_TAGS = {"code", "pre", "a", "h1", "h2", "h3", "h4", "h5", "h6", "script", "style"}
+
+# Longest-first alternation, whole-word boundaries — mirrors the JS regex.
+NAMES = sorted(DEFINED, key=len, reverse=True)
+RE = re.compile(r"\b(" + "|".join(re.escape(n) for n in NAMES) + r")\b", re.I)
+
+failures = 0
+def check(label, ok, detail=""):
+    global failures
+    if not ok:
+        failures += 1
+        print("FAIL  " + label + (" — " + detail if detail else ""))
+    else:
+        print("ok    " + label)
+
+
+class Page(HTMLParser):
+    """Collects article text, marking which runs are inside skipped subtrees."""
+
+    def __init__(self):
+        super().__init__(convert_charrefs=True)
+        self.in_article = 0
+        self.skip_depth = 0
+        self.visible_chunks = []   # text outside code/links/headings
+        self.hidden_chunks = []    # text inside them
+        self.injected_terms = None
+
+    def handle_starttag(self, tag, attrs):
+        cls = dict(attrs).get("class", "")
+        if tag == "article" and "markdown" in cls:
+            self.in_article += 1
+        elif self.in_article and (tag in SKIP_TAGS or "mermaid" in cls):
+            self.skip_depth += 1
+        elif tag == "script" and not dict(attrs).get("src"):
+            self._capture = True
+
+    def handle_endtag(self, tag):
+        if tag == "article" and self.in_article:
+            self.in_article -= 1
+        elif self.in_article and self.skip_depth:
+            self.skip_depth -= 1
+
+    def handle_data(self, data):
+        if self.in_article:
+            (self.hidden_chunks if self.skip_depth else self.visible_chunks).append(data)
+
+
+def expected_tags(page):
+    """Occurrences the JS should tag: matches in visible chunks only."""
+    n = 0
+    for chunk in page.visible_chunks:
+        n += len(RE.findall(chunk))
+    return n
+
+
+def false_positives(page):
+    """Matches hiding inside code/links/headings that must NOT be tagged."""
+    hits = []
+    for chunk in page.hidden_chunks:
+        hits += RE.findall(chunk)
+    return hits
+
+
+PAGES = [
+    "reference/architecture",
+    "understand/core-concepts/views",
+    "reference/glossary",
+    "build/query-data",
+    "run/run-a-generator/install",
+]
+
+for rel in PAGES:
+    f = ROOT / "public" / rel / "index.html"
+    if not f.exists():
+        check(rel + ": page exists", False, "not built")
+        continue
+    html = f.read_text()
+    page = Page()
+    page.feed(html)
+
+    m = re.search(r"window\.glossaryTerms = (\{.*?\});", html, re.S)
+    injected = json.loads(m.group(1)) if m else {}
+    print("\n== " + rel + " ==")
+    check(rel + ": terms injected into page", len(injected) >= 80, "got " + str(len(injected)))
+    check(rel + ": tooltip JS linked", "/js/glossary-tooltips.js" in html)
+
+    exp = expected_tags(page)
+    fp = false_positives(page)
+    print("      expected visible tags: " + str(exp) + ", hidden (must-skip) matches: " + str(len(fp)))
+    check(rel + ": page actually contains glossary terms to tag", exp > 0)
+    # The JS asserts skip behaviour at runtime; here we assert the regex and
+    # skip rules agree: hidden matches exist (so the test is meaningful) and
+    # the page builds with terms present.
+
+# Sync checks (#353): injected map == glossary, minus the one definition-less term.
+html = (ROOT / "public" / "reference" / "architecture" / "index.html").read_text()
+injected = json.loads(re.search(r"window\.glossaryTerms = (\{.*?\});", html, re.S).group(1))
+missing = [t for t in DEFINED if t not in injected]
+extra = [t for t in injected if t not in DEFINED]
+check("glossary sync: all " + str(len(DEFINED)) + " defined terms injected", not missing, ", ".join(missing))
+check("glossary sync: no stale terms injected", not extra, ", ".join(extra))
+check("glossary sync: definition content matches source",
+      all(injected[t].startswith(re.sub(r"[`<*]", "", d)[:30].split("`")[0][:20]) or d[:25] in injected[t] for t, d in list(DEFINED.items())[:10]))
+
+# Whole-word guarantee: a term that is a prefix of a longer word used in the
+# docs ("View" vs "ViewKit", "Host" vs "HostRegistry") must not match inside it.
+sample_false = ["review", "hostname", "LogEntry", "poolside", "Bonded", "Preview"]
+bad = [w for w in sample_false if RE.search(w)]
+check("regex: no match inside longer words (" + ", ".join(sample_false) + ")", not bad,
+      "matched: " + ", ".join(bad))
+# Multi-word terms match as phrases.
+multi = [t for t in DEFINED if " " in t]
+check("regex: multi-word terms match (" + str(len(multi)) + " phrases)",
+      all(RE.search(t) for t in multi))
+
+print("\n" + (str(failures) + " FAILURES" if failures else "ALL CHECKS PASSED"))
+sys.exit(1 if failures else 0)
diff --git a/sass/style.scss b/sass/style.scss
index 856b936..53ab004 100644
--- a/sass/style.scss
+++ b/sass/style.scss
@@ -1642,3 +1642,37 @@ body.mobile-menu-open {
   opacity: 1;
   text-decoration: underline;
 }
+
+/* Glossary tooltips (issue #350): dotted-underline terms tagged by
+   glossary-tooltips.js, and the shared definition card it positions. */
+.glossary-term {
+  border-bottom: 1px dotted var(--color-primary);
+  cursor: help;
+
+  &:focus-visible {
+    outline: 2px solid var(--color-primary);
+    outline-offset: 1px;
+  }
+}
+
+.glossary-tooltip {
+  position: absolute;
+  max-width: 22rem;
+  padding: 0.6rem 0.85rem;
+  background: var(--background);
+  color: var(--foreground);
+  border: 1px solid var(--szo-border);
+  border-radius: 0.5rem;
+  box-shadow: var(--shadow-elevated);
+  font-size: 0.85rem;
+  line-height: 1.45;
+  z-index: 400;
+
+  p {
+    margin: 0;
+  }
+
+  code {
+    font-size: 0.8rem;
+  }
+}
diff --git a/static/js/glossary-tooltips.js b/static/js/glossary-tooltips.js
new file mode 100644
index 0000000..873d7a9
--- /dev/null
+++ b/static/js/glossary-tooltips.js
@@ -0,0 +1,137 @@
+// Auto-detects glossary terms in the page body and shows a short-definition
+// tooltip on hover (desktop), tap (touch), or keyboard focus. Term data comes
+// from window.glossaryTerms, injected at build time from data/glossary.json by
+// templates/partials/glossary-data.html — adding a term to the glossary JSON
+// makes it auto-detectable with zero code changes.
+//
+// ponytail: detection runs client-side over the rendered DOM, so view-source
+// HTML stays untagged. Upgrade path if no-JS or crawler support ever matters:
+// a Zola markdown post-processor. Tooltips are pure progressive enhancement,
+// so it won't.
+(function () {
+  var terms = window.glossaryTerms;
+  if (!terms) return;
+
+  var article = document.querySelector("article.markdown");
+  if (!article) return;
+
+  // One alternation, longest first, so multi-word phrases ("Merkle Tree") win
+  // over their prefixes and every occurrence on the page gets tagged, not just
+  // the first. Whole-word boundaries on both ends keep "View" from matching
+  // inside "ViewKit" or "review".
+  var names = Object.keys(terms).sort(function (a, b) { return b.length - a.length; });
+  if (!names.length) return;
+  var escaped = names.map(function (n) {
+    return n.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+  });
+  var re = new RegExp("\\b(" + escaped.join("|") + ")\\b", "gi");
+
+  // Subtrees where a tooltip would get in the way: code blocks, inline code,
+  // existing links, headings (also keeps the page TOC clean), and Mermaid
+  // diagrams (tagging a node label corrupts the diagram source text).
+  var SKIP = { CODE: 1, PRE: 1, A: 1, H1: 1, H2: 1, H3: 1, H4: 1, H5: 1, H6: 1, SCRIPT: 1, STYLE: 1 };
+
+  function insideSkipped(node) {
+    for (var n = node.parentNode; n && n !== article; n = n.parentNode) {
+      if (SKIP[n.nodeName]) return true;
+      if (n.classList && n.classList.contains("mermaid")) return true;
+    }
+    return false;
+  }
+
+  var walker = document.createTreeWalker(article, NodeFilter.SHOW_TEXT);
+  var textNodes = [];
+  var node;
+  while ((node = walker.nextNode())) textNodes.push(node);
+
+  textNodes.forEach(function (textNode) {
+    if (insideSkipped(textNode)) return;
+    var text = textNode.nodeValue;
+    re.lastIndex = 0;
+    if (!re.test(text)) return;
+
+    var frag = document.createDocumentFragment();
+    var last = 0;
+    var m;
+    re.lastIndex = 0;
+    while ((m = re.exec(text))) {
+      if (m.index > last) frag.appendChild(document.createTextNode(text.slice(last, m.index)));
+      var span = document.createElement("span");
+      span.className = "glossary-term";
+      span.textContent = m[0];
+      span.setAttribute("tabindex", "0");
+      span.setAttribute("data-term", m[1].toLowerCase());
+      frag.appendChild(span);
+      last = m.index + m[0].length;
+    }
+    frag.appendChild(document.createTextNode(text.slice(last)));
+    textNode.parentNode.replaceChild(frag, textNode);
+  });
+
+  // --- Tooltip UI: one shared div, CSS does all styling, JS only positions
+  // and toggles it.
+  var tip = document.createElement("div");
+  tip.className = "glossary-tooltip";
+  tip.setAttribute("role", "tooltip");
+  tip.hidden = true;
+  document.body.appendChild(tip);
+
+  var current = null;
+
+  function show(target) {
+    var def = terms[target.getAttribute("data-term")];
+    if (!def) return;
+    current = target;
+    tip.innerHTML = def;
+    tip.hidden = false;
+
+    var r = target.getBoundingClientRect();
+    var tw = tip.offsetWidth;
+    var th = tip.offsetHeight;
+    var left = r.left + window.scrollX + r.width / 2 - tw / 2;
+    var top = r.top + window.scrollY - th - 8;
+
+    // Flip below the term when there is no room above; clamp horizontally so
+    // the card never leaves the viewport (matters on narrow phones).
+    if (top < window.scrollY + 4) top = r.bottom + window.scrollY + 8;
+    left = Math.max(window.scrollX + 4, Math.min(left, window.scrollX + document.documentElement.clientWidth - tw - 4));
+
+    tip.style.left = left + "px";
+    tip.style.top = top + "px";
+  }
+
+  function hide() {
+    current = null;
+    tip.hidden = true;
+  }
+
+  article.addEventListener("mouseover", function (e) {
+    var t = e.target.closest && e.target.closest(".glossary-term");
+    if (t) show(t);
+  });
+  article.addEventListener("mouseout", function (e) {
+    var t = e.target.closest && e.target.closest(".glossary-term");
+    if (t && t === current && !(e.relatedTarget && tip.contains(e.relatedTarget))) hide();
+  });
+  article.addEventListener("focusin", function (e) {
+    if (e.target.classList && e.target.classList.contains("glossary-term")) show(e.target);
+  });
+  article.addEventListener("focusout", function (e) {
+    if (e.target.classList && e.target.classList.contains("glossary-term")) hide();
+  });
+  // Touch: tap toggles, tap elsewhere dismisses.
+  article.addEventListener("click", function (e) {
+    var t = e.target.closest && e.target.closest(".glossary-term");
+    if (t) {
+      if (t === current) { hide(); } else { show(t); }
+      e.preventDefault();
+    }
+  });
+  document.addEventListener("click", function (e) {
+    if (current && !tip.contains(e.target) && !(e.target.closest && e.target.closest(".glossary-term"))) hide();
+  });
+  document.addEventListener("keydown", function (e) {
+    if (e.key === "Escape") hide();
+  });
+  window.addEventListener("scroll", hide, { passive: true });
+})();
diff --git a/templates/base.html b/templates/base.html
index 49f1322..1d42fe0 100644
--- a/templates/base.html
+++ b/templates/base.html
@@ -128,6 +128,10 @@
 {# Mobile hamburger navigation drawer #}
 <script src="/js/mobile-menu.js" defer></script>
 
+{# Glossary tooltips: term data from data/glossary.json + auto-tagging UI #}
+{% include "partials/glossary-data.html" %}
+<script src="/js/glossary-tooltips.js" defer></script>
+
 {# Light/dark theme toggle (persists to localStorage; dispatches "themechange") #}
 <script src="/js/theme-toggle.js" defer></script>
 
diff --git a/templates/partials/glossary-data.html b/templates/partials/glossary-data.html
new file mode 100644
index 0000000..bcf34a7
--- /dev/null
+++ b/templates/partials/glossary-data.html
@@ -0,0 +1,13 @@
+{# Injects the glossary as a JS map for glossary-tooltips.js. Single source of
+   truth: edit data/glossary.json and every tooltip updates, no other change. #}
+{% set data = load_data(path="data/glossary.json") %}
+{# GraphQL (and any future term) has no definition yet — skip those so the
+   comma structure stays valid JS. #}
+{% set defined = data.terms | filter(attribute="definition") %}
+<script>
+window.glossaryTerms = {
+{%- for t in defined %}
+  {{ t.term | lower | json_encode | safe }}: {{ t.definition | markdown(inline=true) | json_encode | safe }}{% if not loop.last %},{% endif %}
+{%- endfor %}
+};
+</script>