Musical Form
Sonata
A multi-movement work for one or two instruments - and the name of the form's most important internal structure.
Chamber / Solo
"Sonata" does double duty in classical music, which trips up a lot of newcomers. First, it names a genre: a multi-movement piece for a solo instrument (often piano) or a solo instrument with piano accompaniment (a violin sonata, for instance) — smaller in scale than a symphony, but following a similar multi-movement logic.
Second, and more importantly, it names sonata form (also called "sonata-allegro form"): a specific structural template, usually used for a piece's first movement, built in three parts. An exposition introduces two contrasting musical themes. A development section takes fragments of that material and works them through harmonic tension and instability. A recapitulation brings both themes back, now resolved into the same home key, closing the argument the movement opened with.
Sonata form isn't limited to sonatas — it's also the standard first- movement structure for symphonies, string quartets, and concertos, which is why it's worth learning as a pattern rather than as a rule specific to one genre.
Where to start listening: Beethoven's Moonlight Sonata is the most famous piano sonata and a gentle way in, even though its calm opening movement isn't in sonata form itself — its later movements are.