MSG

Usage: MSG <nick|#channel> <message>
This command will allow you to send a private message to a specific user or send a message to an entire channel.
Examples:
/msg Roseanna All I wanna do when I wake up in the morning is see you eyes.
/msg #home Good morning, everybody.
Note: this is a client-side command, so your client might not implement it. See PRIVMSG for the server equivalent.