Face width is length of tooth parallel to axes.

Contact ratio is angle of angle of action and pitch angle.

Pitch circle is the apparent circle that two gears can be taken like smooth cylinders rolling without friction.

Clearance is the radial distance from top of the tooth to the bottom of the tooth space in the mating gear.

Backlash is the tangential space between teeth of mating gears at pitch circles.

Diametral pitch (p) is the number of teeth per unit volume. p =  (Number of Teeth) / (Diameter of Pitch circle)

Circular pitch is the space in pitch circle used by each teeth.

Gear ratio is numbers of teeth of larger gear to smaller gear.

Pressure line is the common normal at the point of contact of mating gears along which the driving tooth exerts force on the driven tooth.

Pressure angle is the angle between the pressure line and common tangent to pitch circles. It is also called angle of obliquity. high pressure angle requires wider base and stronger teeth.