1) Double-strand, closed DNA molecules found in cytoplasm of a variety of bacterial species that generally confer some evolutionary advantage to the host cells.

2) An extrachromosomal ring of DNA that replicates autonomously and is found especially in bacteria.

3) A plasmid is a small, often circular DNA molecule found in bacteria and other cells. Plasmids are separate from the bacterial chromosome and replicate independently of it. They generally carry only a small number of genes, notably some associated with antibiotic resistance. Plasmids may be passed between different bacterial cells.