New Grove dictionary of music and musicians by 
Worship music : a concise dictionary / Edward Foley, editor ; Mark Bangert ... [et al.], consultants.
Harvard dictionary of music / edited by Don Michael Randel.
New Penguin dictionary of music by 
Oxford Music Online
This online resource would be useful in almost every research category. Containing the full text of The New Grove Dictionary of Music and Musicians; The New Grove Dictionary of Opera; The New Grove Dictionary of Jazz; the Oxford Companion to Music; and The Oxford Dictionary of Music.