Revisit means that the owner of the zone may revisit it 23 hours after it took the zone the first time.
On revisits you will get half of the takeover points and 5 minutes of block time.
Revisits can be done an unlimited number of times. (Provided a minimum of 23 hours passes between each visit and the zone is still yours.)
The one who makes 50 revisits within 5 days are given the Revisitor medal.
If you log in at http://www.warded.se/turf/user.php there is a counter who help you count how many revisits you have done, eg "You have made 7 takeovers last 24 hours and 5 revisits the last 5 days."