I've most commonly heard the term midriff used with women's fashion.  

6 hours ago, drpaladin said:

It shouldn't be confused with mid riffs.

The guitarist was interrupted mid riffs by a bottle to his head.

I think I was at that show!

When I think of midriffs, I am reminded of the half-shirts we used to wear back in the 70s-80s for sports.

Noah Hathaway or Boxy from the original Battlestar Galactica(78) or Atreyu from Neverending Story(84) rocking a half-shirt and showing off his midriff. I never missed an episode!

This is a very old term in English, first showing up before 1000 AD.  When it was first used, however, it was spelled midhrif, where hrif meant belly.