Gamma

The curve that maps recorded light to displayed brightness. Get it wrong and the midtones lie about the whole image.

Gamma is the curve between recorded light and displayed brightness, inherited from how eyes and old phosphors actually behave: perception is not linear, so encoding is not either. The curve concentrates its influence in the midtones, exactly where faces, fabrics and most of any image live.

Get it wrong and everything reads as slightly false, shadows lifted into fog or mids sunk into mud, without one identifiable error to point at. Mismatched gamma is the classic why does it look different over there: graded under one curve, displayed under another. The remedy is unglamorous, calibrated displays and explicit tags, and it earns its keep daily.