What is defined as a musical composition for one or more instruments, typically structured in multiple movements?

The term that best fits the definition of a musical composition for one or more instruments, typically structured in multiple movements, is a sonata. A sonata traditionally consists of several contrasting movements, often following a specific structure such as exposition, development, and recapitulation. While the sonata usually features a solo instrument, particularly the piano, it can also refer to compositions for duos and small ensembles.

In comparison, a symphony primarily involves a full orchestra and is composed of multiple movements, but it is broader in scope and more complex in orchestration than a sonata. A concerto specifically highlights a solo instrument with orchestral accompaniment, involving dialogue between the soloist and the orchestra, but it doesn't inherently denote multiple movements in the same way a sonata does. An opera is a theatrical work that combines music, singing, and often dance, focusing on storytelling rather than the instrumental structure implied by the term.

This understanding of the term sonata clarifies why it is the correct choice in this context.