Synonyms: relentless, unrelenting, persistent
Definition: never-ceasing
Usage: the relentless beat of the drums
Similar words: continual
Definition: occurring without interruption; chiefly restricted to what recurs regularly or frequently in a prolonged and closely spaced series
Usage: the continual banging of the shutters
Synonyms: persistent, haunting
Definition: continually recurring to the mind
Usage: haunting memories; the cathedral organ and the distant voices have a haunting beauty- Claudia Cassidy
Similar words: unforgettable
Definition: impossible to forget
Synonyms: unyielding, dogged, dour, tenacious, persistent, pertinacious
Definition: stubbornly unyielding
Usage: dogged persistence; dour determination; the most vocal and pertinacious of all the critics; a mind not gifted to discover truth but tenacious to hold it- T.S.Eliot; men tenacious of opinion
Similar words: unregenerate, obstinate, stubborn
Definition: tenaciously unwilling or marked by tenacious unwillingness to yield