The following are the Terminology of gear

- Pitch circle
- Pitch Circle Diameter (PCD) or Pitch Diameter
- Pitch Point
- Pitch Surface
- Pressure angle or angle of obliquity
- Addendum
- Dedendum
- Addendum circle
- Dedendum circle
- Circular Pitch
- Module
- Clearance
- Back lash
- Path of contact
- Path of approach
- Path of recess
- Arc of contact
- Contact Ration
- Pitch angle
- Base pitch
- Face of tooth
- Flank of tooth
- Total Depth
- Working Depth
- Tooth thickness
- Tooth Space
- Top land
- Face Width
- Profile
- Fillet radius
- Backlash

**Pitch circle**: It is an imaginary circle which by pure rolling action, would give the same motion as the actual gear.**Pitch Circle Diameter (PCD) or Pitch Diameter**: It is the diameter of the pitch circle. The size of the gear is usually specified by the pitch circle diameter. It is also called as pitch diameter.**Pitch Point**: The point of contact of two mating gears pitch circle is known as pitch point.**Pitch Surface**: It is the surface of the rolling disc which the meshing gears have replaced the pitch circle.**Pressure angle or angle of obliquity**: It is the angle between the common normal at the point of contact of two teeth and the common tangent to the pitch point.**Addendum**: It is the radial height of the tooth above pitch circle.**Dedendum**: It is the radial depth of tooth below pitch circle.**Addendum circle**: The circle which passes through the tips of all the tooth is known as addendum circle. This is concentric with the pitch circle.**Dedendum circle**: The circle which passes through the root of all tooth is known as root or dedendum circle.**Circular Pitch**: It is the distance measured along the circumference of the pitch circle from a point on one tooth to a corresponding point on the adjacent tooth.**Module**: It is the length of pitch circle diameter per tooth. It is denoted by ‘m’.**Clearance**: It is the difference between the thickness of a tooth and the width of the toot space in which it meshes.**Backlash**: It is the difference between the thickness of a tooth and the width of the tooth space in which it meshes.**Path of contact**: lt is the path traced by the contact point of a pair of tooth profiles from the beginning of engagement to the en dof engagement.**Path of approach**: The portion of path of contact from the beginning of engagement to the pitch point is known as path of approach.**Path of recess**: The portion from the pitch point to the end of engagement is termed as path of recess.**Arc of contact**: It is the path traced by a point on the pitch circle from the beginning to the end of engagement of a given pair of teeth.**Contact Ratio**: It is the ratio of angle of action to pitch angle and also the ratio of the length of line of action to the base pitch. It is a number which indicates the average number of pairs of teeth in contact.**Pitch angle**: It is the angle subtended by an arc on the pitch circle equal in length to the circular pitch.**Base pitch**: It is the distance measured along the base circle from a point on one tooth to the corresponding point on the adjacent tooth.**Face of tooth**: The surface above the pitch surface is called the face.**Flank of tooth**: the surface below the pitch surface is called the flank.**Total Depth**: It is the radial distance between the addendum and the dedendum circle of gear. It is equal to the sum of the addendum and dedendum.**Working Depth**: It is the radial distance from the addendum circle to the clearance circle. It is equal to the sum of the addendum of the two meshing gears.**Tooth thickness**: It is the width of the tooth measured along the pitch circle.- T
**ooth Space**: It is the width of space between the two adjacent teeth measured along the pitch circle. **Top land**:- It is the surface of the top of the tooth**Face Width**: It is the width of the gear tooth measured parallel to its axis.**Profile**: It is the curve formed by the face and flank of the tooth.**Fillet radius**: It is the radius that connects the root circle to the profile of the tooth.**Backlash**: It is the difference between the tooth space and the tooth thickness, as measured along the pitch circle. Theoretically, the backlash should be zero, but in actual practice some backlash must be allowed to prevent jamming of the teeth due to tooth errors and thermal expansion.