What is a “feature flag”?

Question

High Scalability mentions feature flags here:

5 things toxic to scalability, \"5. Lack of Feature Flags\"

What exactly are feature flags?

Answer 1

A feature flag (also known as feature flipping or feature toggle) is a switch to enable or disable a potentially expensive feature as needed (like, say, when a site is being hammered with unexpected traffic). This'll buy you a little time until you scale up, or until the load spike goes away.

Here's an example from the SWIG documentation.