Passionate kissing for an extended period of time.
Usually involves tongue and can last anywhere from a few minutes to a few hours.
Often also includes removal of certain items of clothing, dry humping, and touching/rubbing/groping of various parts of the body.
After watching the movie at his house we had a make out session for like 3 hours. I let him take off my shirt and bra and feel me up while we dry humped it was so hot I may even give him a handjob next time.