For what portion of the signal cycle does each active element in a push-pull, Class AB amplifier conduct?
Why Amplifier classes are defined by conduction angle, the portion of the input cycle during which the device conducts. Class A conducts the full 360 degrees, Class B exactly 180 degrees, and Class C less than 180. Class AB sits between A and B, so each device is biased slightly into conduction and stays on for more than 180 but less than 360 degrees, which reduces the crossover distortion a pure Class B push-pull stage would show.
Watch out Exactly 180 degrees describes Class B, and the entire cycle describes Class A; the "AB" name is the clue that it falls between those two.
A=360, B=180, C=under 180, so AB is between 180 and 360.