Product-Market Fit means being in a good market with a product that can satisfy that market.
From: Marc Andreesen
Sean Ellis (growth hacker that worked on Dropbox, Xobni) has developed an interesting survey for quantifying the PMF:
"I ask existing users of a product how they would feel if they could no longer use the product. In my experience, achieving product/market fit requires at least 40% of users saying they would be “very disappointed” without your product."