Terminology for Spur Gears
Figure : Spur Gear

Terminology:
Addendum: The radial distance between the Pitch Circle and the top of the teeth.
Arc of Action: Is the arc of the Pitch Circle between the beginning and the end of the engagement of a given pair of teeth.
Arc of Approach: Is the arc of the Pitch Circle between the first point of contact of the gear teeth and the Pitch Point.
Arc of Recession: That arc of the Pitch Circle between the Pitch Point and the last point of contact of the gear teeth.
Backlash: Play between mating teeth.
Base Circle: The circle from which is generated the involute curve upon which the tooth profile is based.
Center Distance: The distance between centers of two gears.
Chordal Addendum: The distance between a chord, passing through the points where the Pitch Circle crosses the tooth profile, and the tooth top.
Chordal Thickness: The thickness of the tooth measured along a chord passing through the points where the Pitch Circle crosses the tooth profile.
Circular Pitch: Millimeter of Pitch Circle circumference per tooth.
Circular Thickness: The thickness of the tooth measured along an arc following the Pitch Circle
Clearance: The distance between the top of a tooth and the bottom of the space into which it fits on the meshing gear.
Contact Ratio: The ratio of the length of the Arc of Action to the Circular Pitch.
Dedendum: The radial distance between the bottom of the tooth to pitch circle.
Diametral Pitch: Teeth per mm of diameter.
Face: The working surface of a gear tooth, located between the pitch diameter and the top of the tooth.
Face Width: The width of the tooth measured parallel to the gear axis.
Flank: The working surface of a gear tooth, located between the pitch diameter and the bottom of the teeth
Gear: The larger of two meshed gears. If both gears are the same size, they are both called “gears”.
Land: The top surface of the tooth.
Line of Action: That line along which the point of contact between gear teeth travels, between the first point of contact and the last.
Module: Millimeter of Pitch Diameter to Teeth.
Pinion: The smaller of two meshed gears.
Pitch Circle: The circle, the radius of which is equal to the distance from the center of the gear to the pitch point.
Diametral pitch: Teeth per millimeter of pitch diameter.
Pitch Point: The point of tangency of the pitch circles of two meshing gears, where the Line of Centers crosses the pitch circles.
Pressure Angle: Angle between the Line of Action and a line perpendicular to the Line of Centers.
Profile Shift: An increase in the Outer Diameter and Root Diameter of a gear, introduced to lower the practical tooth number or acheive a nonstandard Center Distance.
Ratio: Ratio of the numbers of teeth on mating gears.
Root Circle: The circle that passes through the bottom of the tooth spaces.
Root Diameter: The diameter of the Root Circle.
Working Depth: The depth to which a tooth extends into the space between teeth on the mating gear.