theme
A principal musical idea on which a composition or section is based.
In Depth
A theme is a complete musical statement — a melody that serves as the main idea of a piece or movement. Unlike a motif (a short fragment), a theme is a fully formed musical sentence with a beginning, middle, and end. Themes are the building blocks of large-scale musical forms.
In sonata form, the exposition presents two contrasting themes. In theme and variations, a theme is stated and then transformed through successive variations. In a fugue, the theme (called the subject) is the melody that each voice imitates in turn. The ability to create a memorable, flexible theme that can sustain an entire movement is one of the highest compositional skills.
Beethoven's Ode to Joy theme from the Ninth Symphony is so universally recognised that it was adopted as the Anthem of Europe by the Council of Europe in 1972.