1) Something almost too good to be true; incredible; astounding; amazing
2) The result of two unlike parts that combine to create a mutually beneficial relationship.
3) A combination of said parts resulting in magic, electric sparks, fireworks, spontaneous combustion, etc.
1) Last night was totally a setheresa.
2) I think we've kind of become a setheresa.
3) You WISH that was as exciting as a setheresa.