The following are the terms used in Gears. During the design and calculation of the gears these terms are considered.

Gear Terminology 
Pitch Circle
Pitch Circle Is an imaginary circle where two gears will meet as two cylinders with Zero friction.
Pitch circle diameter
It is an important parameter because pitch Circle diameter is used to define the size of the gears. It is simply the diameter of the Pitch Circle.
Pitch point
The point where to Pitch Circle meets is referred as pitch point.
Pitch surface
The surface at the pitch point of the Rolling cylinder replaced by meshing gears.
Pressure angle or obliquity
Angle between the common normal to the point of contact of two gears and the common tangent in the Pitch Circle. It is also called obliquity.
The standard obliquity are 14½° and 20°.
Addendum
The radial distance of a tooth from the Pitch Circle to the top of the tooth is called addendum
Dedendum
The radial distance of a tooth from the Pitch Circle to bottom of the tooth is called dedendum.
Addendum circle
The circle from the top of the tooth which is corecentric to the Pitch Circle
Dedendum circle
A circle from the bottom of the tooth which is Cocentric to the Pitch Circle.
Circular pitch
In the pitch circle distance between two identical points of two consecutive teeth or distance between a point in a tooth and some point on the nearby gear is called circular pitch
Pc=πD/T
Diametral pitch
The ratio between number of teeth and pitch circle diameter
Module
The ratio between pitch circle diameter and number of teeth. It is inverse of diametral pitch
Clearance
The radial Distance from the top of the tooth to the bottom of the tooth in a mating gear is called clearance.
Full depth
Distance between addendum circle and dedendum circle or the sum of addendum and dedendum is called full depth.
Working depth
Distance between addendum Circle to the clearance circle is called working depth.
Tooth thickness
The thickness of tooth along the Pitch Circle is called tooth thickness.
Tooth Space
The space between two consecutive teeth along the Pitch Circle is called tooth space.
Backlash
The difference of toothpaste and tooth thickness is called backless, or in a meeting gear the space between two teeth is called backlash.
Face of tooth
The surface of tooth above the Pitch Circle called face of tooth.
Flank of teeth
The surface below the Pitch Circle is called flank.
Topland
The top surface of a gear is called topland.
Face width
Width of the tooth parallel to the gear rotation axis is called face width.
Gear profile
A Curve formed by the face and the flank of the tooth
Fillet radius
In The root of a gear teeth a fillet is provided. And the radius That connect the root circle and profile of the tooth is called fillet radius.
Path of contact
It is path of point of contact of two mating gears from beginning to the end of the gear meshing.
Arc of contact
In the mating gear the path of a point on the pitch circle is called arc of contact.
Arc of approach
It is a path of a point on pitch Circle of mating gear from the point of engagement to the pitch.
Arc of recess
Path of point on Pitch Circle of two mating gears from the point of disengagement.
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