Noun, sense 2. Someone with whom you basically just get drunk and/or high and have sex, usually on a short term basis. Compare to "your girl" or "a ho", both are higher on the intimacy ladder.
Sense 2: "Are you going to the prom with him?" "Naw. He's just a notch."
Coined on the 26th August 2011, when popular indie games developer Notch (Markus Persson) directed his 300,000 or so twitter followers to visit the website of Grimrock, another indie game, almost immediately crashing it.
Me: Man, that website is gonna get Notched.