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A plasmid is a small DNA molecule that is physically separate from, and can replicate independently of, chromosomal DNA within a cell.

In general, in eukaryotes, episomes are closed circular DNA molecules that are replicated in the nucleus. Viruses are the most common examples of this, such as herpesviruses, adenoviruses, and polyomaviruses. Episomes in eukaryotes behave similarly to plasmids in prokaryotes in that the DNA is stably maintained and replicated with the host cell.

Source is wikipedia.

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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. Is this what you are asking? –  Alan Boyd Apr 10 '13 at 21:28
    
this was exactly what I asked. Thank you very much. –  Armacino Apr 10 '13 at 22:10
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(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.

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