**Pinion :**A pinion is a smaller of the two mating gears.

**Gear :**A gear is the larger of the two mating gears.

**Velocity Ratio :**Ratio of angular velocity of the driving gear to the angular velocity of the driven gear. It is also called speed ratio.

**Transmission Ratio :**Ratio of the angular speed of the first driving gear to the angular speed of the last driven gear in a gear train.

**Pitch Surface :**The pitch surface of the gears are imaginary planes, cylinders or cones that roll together without slipping.

**Pitch Circle :**The pitch circle is the curve of intersection of the pitch surface of revolution and the plane of rotation. It is and imaginary circle that rolls without slipping with the pitch circle of a mating gear.

**Pitch Circle Diameter :**The pitch circle diameter is the diameter of the pitch circle. The pitch circle diameter is denoted by d.

**Pitch Point :**It is a point on the line of centres of two gears at which two pitch circles of mating gears are tangent to each other.

**Top Land :**It is the surface of the top of the gear tooth.

**Bottom Land :**It is surface of the gear between the flanks of adjacent teeth.

**Involute :**An involute is a curve traced by a point on a line as the line rols without slipping on a circle.

**Base Circle :**The base circle is and imaginary circle from which the involute curve of the tooth profile is generated. The base circle of two mating gears are tangent to the pressure line.

**Addendum Circle :**The addendum circle is an imaginary circle that borders the tops of gear teeth in the cross section.

**Addendum :**The addendum is radial distance between pitch and addendum circle. It indicates the height of the tooth above the pitch circle.

**Dedendum Circle :**The dedendum circle is an imaginary circle that borders the bottom of spaces between teeth in the cross section. It is also called root circle.

**Dedendum :**The dedendum is the radial distance between pitch and the dedendum circles. It indicates the depth of the tooth below the pitch circle.

**Clearance :**The clearance is the amount by which the dedendum of a given gear exceeds the addendum of its mating tooth.

**Face of tooth :**The surface of gear tooth between the pitch cylinder and the addendum cylinder is called the face of tooth.

**Flank of tooth :**The surface of the gear tooth between the pitch cylinder and the root cylinder is called flank of tooth.

**Face width :**Face width is the width of the tooth measured parallel to the axis.

**Fillet Radius :**The radius that connects the root circle to the profile of the tooth is called fillet radius.

**Tooth Space :**The width of space between two adjacent teeth measured along the pitch circle is called tooth space.

**Working Depth :**The working depth is the depth of engagement of two gear teeth that is sum of their addendums.

**Whole Depth :**The whole depth is total depth of the tooth space that is sum of the addendum and dedendum.

**Whole depth = Working depth + clearance**

**Centre Distance :**The centre distance is the distance between centres of pitch circles of mating gears.

**Pressure angle :**The pressure angle is the angle which the line of action makes with the common tangent to the pitch circles. It is also called angle of obliquity. It denoted by α.