Install Velocity is the rate at which an app gains downloads over a short window — a momentum signal the store algorithms read as evidence that an app deserves higher ranking.
Install velocity isn't your total download count; it's how fast you're downloading right now relative to your recent baseline. Both app stores lean on it because a sudden, sustained spike is hard to fake and usually means something real changed — a feature, a press hit, a viral moment, a smart update. The algorithm responds by testing the app at a higher rank to see whether the momentum holds.
This is why ASO is rarely a one-and-done edit. A keyword change that lifts you a few spots brings more impressions, which brings more installs, which feeds velocity, which can lift you further — a compounding loop. It also explains why coordinated launches, featuring, and even Apple Search Ads bursts help organic rank: they manufacture the velocity the algorithm is watching for.
Example
After a Today-tab feature, a meditation app's daily installs jump 5× for a week; that velocity spike pushes its search rankings up, and some of the gain sticks even after the feature ends.