noun (n)

• a comment or instruction (usually added)(noun.communication)

  Synonym:
  notation, note
  

  source: wordnet30
• the act of adding notes(noun.act)

  Synonym:
  annotating
  

  source: wordnet30
• 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.(noun)

  source: webster1913