I am returning to the boards after a long hiatus and glad to see this board as most of my activity the past few years has been with SBs, not pros. I'm not familiar with the term unicorn though. What does that mean or imply. Thanks

The dream Sugar.  The perfect NSA relationship.  Think of the best girlfriend you could dream of.  She is hot, kinky, fun as hell, no maintenance, reasonable cost, spends lots of time loving on you and goes crazy in appreciation when you do special for her.   Think 9/9 or above on TER scale.

Try searching the board for past definitions of "Unicorn".  I  posted one a while back, as have others.  

The ladies I know are referring to "hot vs. Crazy matrix"

Depends on where you are.  If you are a swinger, it's a single bi woman.  If in the vanilla world, the hot/crazy matrix defines it as a woman that is an 8 or 9 hot but less than 5 crazy.  It also advises that it doesn't exist, and that if you are dating a woman that falls in this range, that you are dating a dude.