Product-market fit
The point where your product satisfies a real market demand strongly enough that growth and retention start to pull on their own.
Product-market fit is the moment your product clearly serves a market that wants it: users stick around, tell others, and would be genuinely disappointed to lose it. Before fit, growth feels like pushing a boulder; after fit, demand starts pulling you forward and the main problem becomes keeping up.
For a founder it's the milestone that matters more than any feature. Chasing growth tactics before you have fit usually just burns money pouring users into a product they leave. The signal lives in retention and word of mouth, not in vanity signups.
You reach it faster by validating demand before building, not after. Validate a SaaS idea covers how to test whether the demand is real before you commit months to writing code.