A code-level switch that turns a feature on or off for specific users without a redeploy, so shipping a release becomes a config change instead of a deploy event.
Almost every new GitHub feature first ships behind a Scientist-style flag, enabled for staff, then for a percentage of users, then for everyone. The pattern is so embedded that they open-sourced the underlying library, Scientist, in 2016 specifically for safe refactors behind a flag.