In one line
CPI (Cost Per Install) is the cost of a single app install: spend ÷ installs.
Why it matters
It's usually the first cost metric app marketers look at. But a low CPI isn't automatically good — if the user installs and does nothing else, the install was worthless. CPI only tells you how cheaply you acquired users, not whether they're worth anything; pair it with CPA and LTV to know that.