When a guy dates a girl and they only see eachother at the girl's convenience, hence taking him out of the closet once in a while when needed or to just make the guy feel like he matters, then put back in and forgotten about for an extended period of time. This can also be the other way around too.
-Damn i rarely ever see Mark and Tracy together
-Thats because he's just her closet boyfriend
The mutual boyfriend that only appears when another male becomes interested in the female
Bob: So ya wanna go out this friday?
Sarah: Sure! That soun-......
Jim: Hey hot stuff!
Sarah: Hey! Bob this is my boyfriend Jim.
Bob: Ummmm, WTF?
Jim is a Closet Boyfriend