Autonomously replicating extra-chromosomal circular DNA molecules, distinct from the normal bacterial genome and nonessential for cell survival under nonselective conditions. Some plasmids are capable of integrating into the host genome. A number of artificially constructed plasmids are used as cloning vectors.
www.ornl.gov/TechResources/Human_Genome/glossary/glossary.html

Independent, free-floating circular piece of DNA in a bacterium, capable of making copies of itself in the host cell. Plasmids can be used in recombinant DNA experiments to clone genes from other organisms and make large quantities of their DNA.
ucbiotech.org/glossary/

A small circular piece of DNA in bacteria that resembles the bacterial circular chromosome, but is dispensable. Some bacterial strains contain many plasmids and some contain none. Plasmids are often used in genetic engineering as cloning vectors.
www.oznet.ksu.edu/biotech/glossary.htm

A small self-replicating ring of DNA found in many bacteria and some yeasts. They are widely used in genetic modification because they are able to pass easily from one cell to another.
www.ncbe.reading.ac.uk/NCBE/GMFOOD/igdglossary.html

An extrachromosomal genetic element found in bacteria, not essential for growth. Usually contains genetic information for resistance to an antimicrobial agent or for degradation of additional substrates.
www.erc.montana.edu/Res-Lib99-SW/glossary/gmicro.html

A genetic particle physically separate from the chromosome of the host cell (chiefly bacterial) that can stably function and replicate; not essential to the cell's basic functioning. Synonyms: extrachromosomal element, extrachromosomal genetic element, paragene (Dirckx, 1997).
www.med.uwo.ca/ecosystemhealth/education/glossary.htm

A DNA molecule that can replicate independently of the chromosome; often used in cell regulation
student.biology.arizona.edu/honors2000/group10/Glossary.htm

Circular loop of DNA in prokaryotes. Eukaryotic DNA is organized into chromosomes.
www.ucmp.berkeley.edu/glossary/gloss4cell.html