Retention Curve
A retention curve plots the percentage of a cohort still active over time, from day one onward. You take everyone who signed up in a given week or month, then track what fraction is still using the product after one week, one month, three months, and so on. The shape of the line tells you almost everything about product-market fit.
What a founder is looking for is a curve that flattens, a point where it stops declining and holds steady, because that plateau is your true retained base, the customers the product genuinely works for. A curve that keeps sliding toward zero means you have a leaky product no amount of acquisition will fix; you're filling a bucket with no bottom. The height of the flattening line is your ceiling. Improving retention isn't about the early steep drop, which is partly tyre-kickers leaving; it's about lifting and flattening that plateau.