In its standard meaning, sonata form is a 3-part form in music composition, in which the second and third parts are closely linked so as to imply a 2-part organization. The three parts are called exposition, which presents the principal theme, establishes the tonic key, and modulates to some other closely related key; development, in which the exposition theme is fragmented and reworked into new combinations and sequences, and recapitulation, which reintroduces the exposition theme as it was originally played. (The 2-part organization appears most clearly when the exposition is played twice.)
Sonata form refers to the form of a single movement rather than to the whole of a 3- or 4-movement sonata, symphony, or work of chamber music. It is sometimes called first movement form or sonata allegro form. The term, sonata form was the invention of A.B. Marx, a music theorist/writer of the 19th century who devoted is life to the deification of Beethoven and establishing the myth of Beethoven’s musical supremacy. This is why sonata form as it is commonly known is largely a generalization of the procedures of Beethoven before 1812. In Marx’s defense, it was intended above all as an aid to composition. The method of defining a form by taking the works of a famous composer as models is rightfully discredited today.