An event invitation sent to someone solely for the reason that one feels obligated to do so.
For example, there are three roommates
and you know two of them well. You may not know the third person as well as the other two--or you just don't like them--yet you feel obligated to invite that person too so things won't get awkward.
Cory: So who's on the invite list?
Bob: . . . Amber, Emily, Roxy--
Cory: I thought you didn't like Roxy . . . ?
Bob: I don't. But they're roommates, so I just sent her a courtesy invite so that things wouldn't get awkward.