Main entry: relentless, unrelenting, persistent
Definition: never-ceasing
Usage: the relentless beat of the drums
Main entry: persistent, haunting
Definition: continually recurring to the mind
Usage: haunting memories; the cathedral organ and the distant voices have a haunting beauty- Claudia Cassidy
Main entry: lasting, persistent
Definition: retained; not shed
Usage: persistent leaves remain attached past maturity; the persistent gills of fishes
Main entry: 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