AMG: All Music Guide: Music Glossary
Categorised by:

1. Arrangements, Forms, and Structure
2. Composition and Harmony
3. Effects, Embellishments and Ornaments
4. Genres
5. Nationality
6. Recording, Processing, Sessions and Sound
7. Vocal Performance, People, Technique
8. Beat, Meter, Rhythm, and Tempo
9. Dance
10. Ensembles, Combos, Groups and Bands
11. Instruments
12. Pitch, Notation, Mensuration and Dynamics
13. Styles

Creative Online Music Dictionary of Musical Terms

hnh.com: The Classical Music Website: Online Music Glossary

!FORTISIMO! Music Dictionary

Mysheetmusic Online Music Dictionary

Online Dictionaries
This site lists online dictionaries in almost every language under the sun,
could come in handy for those Norwegian and Scandinavian pieces of music.

Sony/W.W. Norton: Essentials of Music: Online Glossary

Spindrift Thesaurus of Musical Terms
This archive of musical terminology gets special mention. It's the only online reference I've found with the definition of "con islancio".