1.Definition: continually recurring to the mind

Related Adjective(s):haunting

2.Definition: never-ceasing

Related Adjective(s):relentless, unrelenting

3.Definition: retained; not shed

"Persistent leaves remain attached past maturity", "The persistent gills of fishes"

Related Adjective(s):lasting

4.Definition: stubbornly unyielding

Related Adjective(s):dogged, dour, pertinacious, tenacious, unyielding