Sex that is purely used after a break up or disagreement between partners, or lovers.
Where parties engage in sexual relations, immediately after a break up or painful disagreement. More of a final attempt at reconciliation in some instances, but always is precluded by an intense disagreement, and/or argument.
The concept is that the sexual gratification increases, is more intense, than normal sex is. This usually follows a break up.
This is more of a physical reaction, that is supposed to be more intense, due to the knowledge that it won't be happening again with this particular party.
Noteworthy: Makeup Sex is generally something to make fun of, afterwards. It isn't about the relationship, but about getting one last one in, and so in theory, is more physical than emotional, though emotions play a role in it.