Plasmid. Definition. noun, plural: plasmids. A linear or circular double-stranded DNA that is capable of replicating independently of the chromosomal DNA. Supplement. Certain plasmids are able to insert themselves into the chromosomes particularly in regions where there is a common sequence of nucleotides. linear or circular double-stranded DNA that is capable of replicating independently of the chromosomal DNA. Supplement. Certain plasmids are able to insert themselves into the chromosomes particularly in regions where there is a common sequence of nucleotides.