Plasmid – An extrachromosonal genetic element that is not essential for growth present within some bacterial cells.
A plasmid is made up of DNA that can exist ad replicate independantly of the chromosome. Plasmids provide genetic instructions for certain cell activities (eg antibiotic resistance). They can be transfered from cell to cell in a bacterial colony using sex factors to commence conjugation.
Plasmids are widely used as vectors to produce recombinant DNA for gene cloning.