A motive is the smallest identifiable musical idea. Motives may be developed into phrases, periods, full melodic ideas, and even entire large scale pieces.

For instance, the motive of Beethoven's fifth symphony is introduced first thing. The opening four notes (three eighth notes followed by a quarter note under fermata) outline everything that happens for the rest of ths symphony. The entire harmonic and melodic structure can be traced back to the minor third interval in the opening statement.

To begin a new piece, a composer will often times decide on a motivic subject and then develop it through exposition and improvisation (sometimes on paper, other times at the keyboard) until a solid, coherent thread of development is acheived.