A melodic or, sometimes a harmonic idea presented in a musical form.

A melody that assumes importance in the development of a composition due to its continued use.

The most important melody at any specific time in a musical work. There can be one main theme in a work, or many themes.

(subject): The musical material that forms the basic element in the structure of a composition. It usually has a recognisable (leading) melody and is a complete musical expression in itself (unlike the smaller unit motif). A sonata movement has two main subjects (or subject groups) whereas a fugue usually has a single subject. In a sonata movement, the theme would be the leading phrase. As a recognisable entity, a theme could be used to identify a work (the first movement of the Eroica is well-known with its first subject only).