Advanced primer ignition, a type of blowback operation for firearms that ignites the primer when the bolt is still moving with the goal of increasing rate of fire and decreasing felt recoil.
Petroleum industry nomenclature deriving from American Petroleum Institute
API gravity, a measure of the density of petroleum
API number, a unique identifier applied to each petroleum exploration or production well drilled in the United States
API unit, a standard measure of natural gamma radiation measured in a borehole
Analytical Profile Index, a miniaturized panel of biochemical tests compiled for identification of groups of closely related bacteria