The Collaborative International Dictionary of English v.0.48:

Footnote \Foot"note`\, n.
   A note of reference or comment at the foot[4] of a page.
   [1913 Webster]

WordNet (r) 3.0 (2006):

footnote
    n 1: a printed note placed below the text on a printed page
         [syn: footnote, footer]
    v 1: add explanatory notes to or supply with critical comments;
         "The scholar annotated the early edition of a famous novel"
         [syn: annotate, footnote]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "footnote":
   adversaria, aide-memoire, annotation, apparatus criticus, comment,
   commentary, commentation, docket, entry, exegesis, gloss, item,
   jotting, marginal note, marginalia, memo, memoir, memorandum,
   memorial, minutes, notation, note, note of explanation, register,
   registry, reminder, scholia, scholium, word of explanation