| The ubiquitous friend used by two people in a relationship as a way not to become bored with each other, while at the same time continuing to go out. This third person must be OVERLY nice, probably male (women generally don't put up with the kind of bullshit the consensual third wheel has to put up with), and a good conversationalist.
This person basically acts as an emotional crutch for the other two parties involved, and keeps the relationship going long after it should be over by giving the two people something to do during the day so they can have an excuse to have sex at night.
If you find yourself in such a situation, point it out two the other two people immediately! (and politely, one would hope.) The end result of such a situation is that the third wheel is forced to decide between the other two parties when the inevitable break up occurs.
See: Bitterness | 
