Is episome a plasmid or a virus?
(Reposting my comment as an answer since it seems to be what was required.)

A DNA molecule that replicates independently of chromosomal DNA is an episome. By this definition a plasmid is (usually) an episome. If a plasmid integrates into a chromosome by some mechanism (as for example in Hfr strains of E. coli where the F plasmid is integrated) the plasmid loses its episomal status. 

The term "episome" was first coined by Joshua Lederberg to describe the nature of the F element in E. coli.