You can send private messages to other users that only the two of you can see.
Starting a Conversation
/query <nickname>
This opens a PM tab. Alternatively, send a single message:
/msg <nickname> <message>
Using the User List
Right-click a nickname in the user list on the right and select Query to open a PM tab. You can also double-click a nickname to open a PM directly.
A private message keeps that user list too: it shows the two of you under Participants, with a dot saying whether the other person is online, away or not connected. Right-click them there for Whois, Ignore, Add to Contacts and a P2P session.
For a lightweight message that does not open a PM tab on the recipient's side, choose Send Notice from a nickname context menu or use
/notice.
See Also
Msg Command · Query Command · Notice Command · Notices · User List