A plasmid is defined as a replicon, or replicating piece of DNA, that is stably inherited in an extrachromosomal state. In older literature, the term episome was used for plasmids capable of integration into the chromosome, but this term has largely gone into disuse. It typically exists as a covalently closed circular piece of double stranded DNA that has the capability of replicating autonomously and it is this property that leads to its isolation and physical recognition. The closed covalent nature of their structure allows them to be separated from chromosomal DNA by either gel electrophoresis or cesium chloride buoyant density gradients. The broad host range of some plasmids is at least in part explained by their multiple replication systems that allow them to function in a variety of dissimilar hosts.
All plasmids fall into only one of the many existing incompatibility groups.
Two plasmids are incompatible if either is less stable in the presence of the other than it was by itself. There are more than 30 incompatibility groups thus far described with no upper limit in sight. Incompatibility, whose genotypic designation is inc, is often a necessary consequence of a plasmid's desire to maintain a certain copy number in the cell. If plasmids of a given incompatibility group have a certain copy number that they attempt to maintain, then a competition will result when two plasmids of the same incompatibility group are found in the same cell. Whichever plasmid is able to replicate faster, or has some other advantage, will be represented to a disproportionate degree among the copies allowed by the incompatibility system.