Product/Market Fit is the degree to which a product satisfies a strong market demand.
Sean Ellis defines Product Market fit as follows in his blog post "The Startup Pyramid":
"I’ve tried to make the concept less abstract by offering a specific metric for determining product/market fit. 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. "