An adjustable strength of feedback to a gyro or flight stabilizer.  Determines aggressiveness of control action.  Used in RC helicopters to stop the tail from exhibiting un-commanded changes in direction due to disc loading and/or wind.  Also a term used in fixed wing aircraft stabilizers to tune control action in the roll, pitch, and yaw axes.   Gain settings that are too high will induce movement rather than dampen them.  Gains that are too low will be ineffective.