The Collaborative International Dictionary of English v.0.48:

Maestro \Ma*es"tro\ (m[imac]s"tr[-o]; It. m[aum]*[asl]s"tr[-o]),
   n. [It., fr. L. magister. See Master.]
   A master in any art, especially in music; a composer or
   orchestra conductor.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

maestro
    n 1: an artist of consummate skill; "a master of the violin";
         "one of the old masters" [syn: maestro, master]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "maestro":
   abecedarian, artist, artiste, certified teacher, concert artist,
   docent, doctor, dominie, don, educationist, educator, executant,
   fellow, guide, guru, instructor, interpreter, master, melamed,
   mentor, minstrel, minstrelsy, mullah, music maker, musician,
   pandit, pedagogist, pedagogue, performer, player, preceptor,
   professor, pundit, rabbi, schoolkeeper, schoolmaster,
   schoolteacher, soloist, starets, teacher, tunester, virtuosa,
   virtuoso