IRC command found in many clients for input diversion. Typically, everything you type go to channel, but if you type /query user, everything will go to that user instead. Handy if you want to talk with someone for extended period and don't wish to overuse /msg or /action. =)

In some clients, this will open a new window or screen in which you can chat with that user, and it can be closed if you want to end the query. In some others, query can be ended with just /query.