A
AB - Binary
A piece of music in two sections which may be contrasted in some way. The first section is called ‘A’ and the second section ‘B’.
ABA - Ternary
A piece of music in three sections where the third section is exactly or almost exactly a repetition of the first section, the second section (‘B’) being different. The term Ternary still applies if there is an additional ‘A’ Section e.g. AABA.
ABACA - Rondo
A piece with any number of sections where one idea (usually the first, ‘A’) keeps recurring.
Accent
An emphasis or stress on a particular note or group of notes.
Accompaniment
These days often applies to instruments or voices playing a subservient role to a soloist or 'the tune'. In the (18 this was not the case, the accompaniment having equal importance.
Arrangement
Adaptation of music for another medium than that for which it was originally composed, e.g. Scott Joplin's 'The Entertainer' for recorder quartet.