A single person (or person without their significant other present) who goes somewhere with a couple. In turn usually making it awkward for both the couple and themselves when the couple wants to do things such as making out.
In america, I think you call it a third wheel.
Kelso : "Hey guys! Wanna play monopoly? Wanna watch wrestling?"
Hyde and Jackie : No Kelso. Go away.
Kelso : But... I wanna hang out with you guys!
Hyde : Don't make me poke you in the eye!
Hyde to Jackie : Now we can finally make out without that gooseberry watching us!