Product-Market Fit
Also called PMF
The point where you have a product a specific market clearly wants, so much that people pull it from you rather than you pushing it on them. Before it, growth is a grind of convincing. After it, the main problem becomes keeping up with demand.
Why it matters
It is the single milestone most early startups are really chasing. Almost nothing else, hiring, fundraising, scaling, works before it, and most things get easier after. Knowing whether you have it changes what you should spend the next quarter doing.