In a malware context. Command and Control (C2) is a collection of methods and software used to communicate with and control infected hosts.
Many C2 solutions are documented in the C2 Matrix: https://howto.thec2matrix.com/ https://docs.google.com/spreadsheets/d/1b4mUxa6cDQuTV2BPC6aA-GR4zGZi0ooPYtBe4IgPsSc/edit#gid=0