Extrachromosomal DNA refers to the genetic material that exists outside the main chromosome of an organism. It is also known as an episome or plasmid. Episomes are considered extrachromosomal DNA as they can integrate into the chromosome or replicate independently. Plasmids are small circular DNA molecules that exist independently from the chromosomal DNA and are primarily found in bacteria and archaea. Extrachromosomal DNA can also be referred to as an extragenomic element or mobile genetic element. This DNA is essential for bacteria to evolve and adapt to new environments through horizontal gene transfer. Overall, extrachromosomal DNA plays a crucial role in the genetic diversity and evolution of organisms.