What, if any, is the difference between covalently closed circular DNA and plasmids?
As a descriptive term, we can take covalently closed, circular DNA (cccDNA) at face value: any DNA that is circular, with the ends joined by covalent bonds, can be described as cccDNA. Thus, plasmids are a type of cccDNA, but not all cccDNA are plasmids. Textbooks and lecturers describe plasmids as cccDNA for the purposes of DNA topology. This usage also appears in literature. cccDNA can also refer to circularized viral DNA, which is another type of episome. Note that the linked to papers are merely examples of how the term is used, not authoritative definitions.