Here is a list of free, online Music Dictionaries:

Virginia Tech Multimedia Music Dictionary 
Created by Richard Cole - Virginia Tech Department of Music and Ed Schwartz - Virginia Tech New Media Center

Select a letter of the alphabet from the list below to look up any musical term. The terms starting with that letter will appear on the left side of the screen. 

Music Dictionary 
!FORTISSIMO! A Program for Musical Development -- Hitting all the right notes, this web site soars into excellence with its simple design and clearly-written content. Students can explore the terminology of music, including words used in musical notation. They can also use this site to find short biographies of famous composers. Various musical instruments are pictured, along with a brief annotation for each one. As is appropriate for a site about music, audio clips of some of these instruments are available.

Music Dictionary Online 
Our Music Dictionary Online explains words and terms found in musical scores, books about music, web sites on music, notes on CDs or in concert programmes. We have provided many links to supplementary material in our own Music Theory Online resource, our Musical e-Monographs and external websites. Where words have changed their meanings or usage we have tried to provide some information on this too.

http://dictionnaire.metronimo.com/index.php 
Les définitions proviennent du dictionnaire pratique et historique de la musique par Marie BOBILLIER, - en littérature Michel BRENET, - édité en 1926. Il n'y a donc pas de termes en rapport avec  le jazz, le rock, la musique contemporaine, etc. Si vous ne trouvez pas ici les renseignements dont vous avez besoin, consultez l'annuaire de Métronimo ou l'annuaire dmoz.

Austin Symphony Glossary of Musical Terms