Glossary
Also: Feature toggle
A switch in the code that turns functionality on and off at runtime, without shipping a new release.
Feature flags decouple two things that usually coincide: shipping code and releasing a feature. Unfinished code can travel to production safely, and a new feature can be opened to individual customers first.
For migrations they are the tool of choice: the new path first serves one percent of traffic, then ten, and if in doubt you close it again in seconds.