Is the arc of the Pitch Circle between the first point of contact of the gear teeth and the Pitch Point. 

Play between mating teeth.

The distance between centers of two gears.

The distance between the top of a tooth and the bottom of the space into which it fits on the meshing gear.

The ratio of the length of the Arc of Action to the Circular Pitch. 

Teeth per mm of diameter.

The working surface of a gear tooth, located between the pitch diameter and the top of the tooth.

The width of the tooth measured parallel to the gear axis.

The working surface of a gear tooth, located between the pitch diameter and the bottom of the teeth

Larger of the two meshing gears is called wheel.