Definitions for "Class D"
A Class D audio amplifier has significantly higher power efficiency than traditional linear amplifiers.
operation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle. Since the output devices are either completely on or completely off they do not theoretically dissipate any power. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%.
So-called digital amplifiers, not because they’re driven directly by binary data but because the output circuits are always either fully on or off. Operation is similar to that of a switch-mode power supply, except that audio signals control the switching action. Efficiency is very high, making Class D amps desirable for compact and battery-powered portable gear.
Keywords:  girder, cross, mounted