Circular DNA is DNA that forms a closed loop and has no ends. Examples include: Plasmids, mobile genetic elements; cccDNA, formed by some viruses inside cell nuclei; Circular bacterial chromosomes; Mitochondrial DNA (mtDNA); Chloroplast DNA (cpDNA), and that of other plastids; Extrachromosomal circular DNA (eccDNA).
Grossi, R., Iliopoulos, C.S., Mercas, R. et al. Circular sequence comparison: algorithms and applications. Algorithms Mol Biol 11, 12 (2016). https://doi.org/10.1186/s13015-016-0076-6