You can send a private reply in a topic, which only allows the responded user to view the message content.
1. Go into the topic you want to post the reply.
2. Click 
under the post to open the Private Post form.
3. Enter your message and send it as described in the Post an advanced reply section.
Note
Private posts will not be checked for approval when it is posted in a topic that has the 'Posts moderation' enabled. Private posts are displayed with the 'Post private!' label.