Source : Free On-Line Dictionary of Computing

Client To Client Protocol
     
         (CTCP) A type of {protocol} created to allow
        structured data such as {font} information to be exchanged
        between users on {IRC}.  It is also used to send a query to a
        user.  The available CTCP commands include VERSION, FINGER,
        DCC CHAT, DCC SEND, TIME, PING, ECHO, CLIENTINFO.  Some
        commands are not available on some IRC {client} software.
     
        (1995-04-12)