a date-like activity or activities (i.e. movies, dinner) performed by a guy and a girl together, who have no interest in each other and no intent to date. These activities are performed strictly as friends, although these not-dates can be segues into the awkward phase before actual dating commences.
For other people, distinguishing between a date and a not-date is difficult, if not impossible. Thus, those who are on a not-date are often confused by other people that they are dating or vice versa.
In the base metaphor, a not-date is the equivalent of sitting in the bleachers and watching the game.
Ryan: "So then Claire and I went to dinner and then watched a movie at my place."
Jake: "All right! So did you hit that?"
Ryan: "Hell no. It was a not-date."
(at a restaurant)
Waiter: "Okay, you're total's going to be $23.44."
Jimmy: "Can you make those two separate orders?"
Waiter: "Sorry about that, I thought that you were on a date."
Jimmy: "No, we're friends. This is a not-date."
A not-date is a sad excuse for people who can't decide whether they are going to get some or not. Often misused for DWOB.
That guy is so sad, going on all of those not-dates, I mean, DWOBs