Not sure if you're referring to the actual definition. It's basically "no two participants from different pairs in the matching would prefer each other to their current matches." It seems an elegant definition to me.

This obviously doesn't factor in most of the complexity of romantic relationships: not knowing your own preferences, evolving preferences, incomplete/imperfect information in general, etc. Plus it assumes a global 1:1 matching across two categories, and people can of course be LGBT or choose to be single.