Initially the name of a type of battle or tournament from the Morkom System, "Mortal Kombat" has been identified as a mysterious, reality-bending phenomenon that occurs when Morkom System natives or specially-trained non-Morkomians invoke it.
When Mortal Kombat is invoked, the invoker and their designated opponent are walled off in a powerful supernatural barrier that bars entry or exit by anything. The barrier doesn't lower until one of the "kombatants" wins 3 rounds of combat. At the end of each round of combat, both kombatants are regenerated to pristine health, before the next round of combat.
It is dangerous for non-Morkomians to participate in Mortal Kombat: loss of Mortal Kombat for an unprepared non-Morkomian can result in the loss of their soul.
Winona Fay, while being trained by the Lin Kuei, was taught the intricacies of Mortal Kombat by Sub-Zero, a veteran of Mortal Kombat, allowing Winona to both invoke it, and retain her soul should she lose.