- persistent, relentless, unrelenting - never-ceasing; "the relentless beat of the drums"
Antonym: sporadic (indirect, via continual)
- haunting, persistent - continually recurring to the mind; "haunting memories"; "the cathedral organ and the distant voices have a haunting beauty"- Claudia Cassidy
Antonym: forgettable (indirect, via unforgettable)
- persistent, lasting - retained; not shed; "persistent leaves remain attached past maturity"; "the persistent gills of fishes"
Antonyms: caducous, shed, deciduous