CHANNEL MODE k

Usage: MODE <channel> +|-k [<key>]
k - Key
Sets the channel so only users who specify the channel's key (password) as the second parameter of the join command can enter the channel. If the key to the channel #CoolChat were bagel then to join one would use the command /JOIN #CoolChat bagel. Unlike passwords, keys are not case sensitive.
Privilege: channel operator
Examples:
/mode #movies +k secret173
See also: CMODE +j