What is the definition of retrovirus?

Retrovirus is any of a group of RNA viruses which insert a DNA copy of their genome into the host cell in order to replicate, e.g. HIV.
A virus that is composed not of DNA but of RNA. Retroviruses have an enzyme, called reverse transcriptase, that gives them the unique property of transcribing their RNA into DNA after entering a cell. Retroviridae is a family of enveloped viruses that replicate in a host cell through the process of reverse transcription.