WordNet (r) 3.0 (2006):

key signature
    n 1: the sharps or flats that follow the clef and indicate the
         key [syn: key signature, signature]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "key signature":
   accent, accent mark, bar, cancel, character, custos, direct,
   dominant, dot, expression mark, fermata, hold, key, keynote, lead,
   ligature, major, major key, mark, measure, mediant,
   metronomic mark, minor, notation, pause, pedal point, presa, segno,
   sign, signature, slur, subdominant, submediant, subtonic,
   supertonic, swell, symbol, tempo mark, tie, time signature,
   tonality, tonic, tonic key, vinculum