"Product Market Fit" is a term coined by Marc Andreessen in 2011.
It is a state where a product is successful in the market, and the market is successful with the product.
Source: https://en.wikipedia.org/wiki/Product%E2%80%93market_fit
How to measure Product Market Fit?
On a scale of 1 to 10 (1 being "don't care", 10 being "OMG NOOOOOOOOOOO!!!"), how would you feel if you could no longer use the product?