The Collaborative International Dictionary of English v.0.48:

Annotation \An`no*ta"tion\, n. [L. annotatio: cf. F.
   annotation.]
   A note, added by way of comment, or explanation; -- usually
   in the plural; as, annotations on ancient authors, or on a
   word or a passage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

annotation
    n 1: a comment or instruction (usually added); "his notes were
         appended at the end of the article"; "he added a short
         notation to the address on the envelope" [syn: note,
         annotation, notation]
    2: the act of adding notes [syn: annotation, annotating]

The Free On-line Dictionary of Computing (19 January 2023):

annotation
annotate

   1.  Extra information associated with a
   particular point in a document or program.  Annotations may be
   added either by a compiler or by the programmer.  They are
   not usually essential to the correct function of the program
   but give hints to improve performance.

   2.  A new commentary node linked to an existing
   node.  If readers, as well as authors, can annotate nodes,
   then they can immediately provide feedback if the information
   is misleading, out of date or plain wrong.

   (1995-11-26)