Product-market fit (PMF) is the point where your product satisfies a strong market demand. Customers are getting real value, returning on their own, and telling others, and growth starts pulling you forward instead of you pushing it uphill.
Marc Andreessen defined it as "being in a good market with a product that can satisfy that market." Simple to state, hard to achieve.
How you know you have it
The clearest signal is retention. If users sign up, get value, and come back repeatedly, something's working. If they sign up, poke around, and disappear, you don't have PMF yet.
Sean Ellis's "40% test" is a useful measure: ask users how they'd feel if they could no longer use your product. If 40% or more say "very disappointed," you're likely close. Track this feedback in FeatureOS Boards to spot patterns across your user base.
Why it's not permanent
PMF isn't binary and it doesn't stay fixed. You can have strong fit with an early niche and lose it when you try to expand. You can also find it in a segment you didn't originally target.
Most teams get there through iteration: customer conversations, discarded ideas, and eventually finding the version of the product where things click. The milestone is real. The path rarely goes as planned.