Footnotes
Put a reference like this[1] in the text and define it anywhere in the page. All definitions are collected at the bottom, numbered in order of first use, with a link back to the text.
A second note[2] shows that definitions may have several paragraphs, and a named label is fine.[1:1] Reusing a label adds another back link.
Footnotes inside an embedded page keep working: ids get a unique suffix so they never clash with the host page.