ostinato

in music, an ostinato (derived from Italian: stubborn, compare English: 'obstinate') is a motif or phrase, that persistently repeats in the same musical voice, usually at the same pitch. The best-known ostinato-based piece may be Ravel's Boléro.^[1]

an ostinato is always a succession of equal sounds, where each note has the same weight or stress. 
The repeating idea may be a rhythmic pattern, part of a tune, or a complete melodyin itself.^[2] 
Both ostinatos and ostinati are accepted English plural forms, the latter reflecting the word's Italian etymology. 

strictly speaking, ostinati should have exact repetition, but in common usage, the term covers repetition with variation 
and development, such as the alteration of an ostinato line to fit changing harmonies or keys. 
if the cadence may be regarded as the cradle of tonality, the ostinato patterns can be considered the playground in 
which it grew strong and self-confident.

—edward e. lewinsky