When one is in a relationship that both people know is ending and one of the two people starts to look for another relationship. This could be described as cheating but is usually not recognized as such as both people know that the end of the current relationship is emminent.
"Did I see you with Carrie yesterday?"
"Dude, I thought you were still dating Melissa?"
"I am but it's pretty much over and I'm overlapping"
When two people have hooked up with at least one person in common, usually used when the number is a lot more.
Thaddeus: Bro, I finally got to fuck Jocinda last night!
Tyrel: That's not coo Thad. We have too much overlap already!
Thaddeus: Come on man. Eunjeung, Latoya, and Ana...so with Jocinda that's 4. You're just jealous that you couldn't get it up and she never came back.
Tyrel: Fuck you man, what about Puja and Juana? That's 6 hoes dude. That's too much.
Thaddeus: Oh shit, you're right man. Too much fuckin' overlap. We gotta stop.
noun - the occurance of dating two people at one time, when one relationship is ending and anther is beginning.
verb - to have two dating relationships that are beginning and ending concurrently.
1. Did you date Peter and Jon at the same time?
Not really. It was an overlap.
2. Peter and I broke up after I met Jon. He was my overlap.
3. You dated a girl named SherriLynne?
NO, i dated two girls, Sherri and Lynne. It was an overlap.