Defined in msdn:
A cmdlet is a lightweight command that is used in the Windows PowerShell environment.
The Windows PowerShell runtime invokes these cmdlets within the context of automation scripts that are provided at the command line.
The Windows PowerShell runtime also invokes them programmatically through Windows PowerShell APIs.
Cmdlets and its frequently used terms are
[Cmdlet(VerbsCommon.Get, "Proc")] public class GetProc : Cmdlet