motif
A short musical idea — a melodic, rhythmic, or harmonic fragment — that recurs throughout a piece.
In Depth
A motif (or motive) is the smallest identifiable musical idea. It might be as short as two notes — the opening four notes of Beethoven's Fifth Symphony (da-da-da-DUM) form one of the most famous motifs in music history. Unlike a theme, which is a complete musical sentence, a motif is a fragment that gains meaning through repetition and development.
Motifs unify a composition by appearing in different contexts — transposed, inverted, augmented, diminished, or combined with other motifs. Wagner developed this technique extensively with his leitmotifs — recurring musical themes associated with specific characters, objects, or ideas in his operas. Film composers like John Williams inherited this technique directly from Wagner.
The four-note motif that opens Beethoven's Fifth Symphony was used as a victory signal by the BBC during World War II — its rhythm (short-short-short-long) matches the Morse code for V.