Retention curve
Also said as Day-1 retention, D1, D7, D30, Churn, Cohort retention
The share of players who come back after a given number of days, starting from those who first played on the same day. Day-1 retention is the share who return the next day. Churn is the opposite: those who leave.
It tells you whether people want your game, more honestly than downloads do. Curves differ a lot by genre and platform, and any "good" number you hear will be a vendor's average at some date, so compare your game with its own last version. Retention is also not the same as enjoyment. You can raise it with streaks and loss aversion and be left with people who feel trapped.
Say it
“Day 1 dropped 5 points after the new tutorial. Check the first ten minutes for where people quit, and fix that before we touch anything later in the game.”
Not to be confused with engagement
Retention is whether they come back at all. Engagement is how much they do once they're here.