1. The length of a helical tooth in one revolution, if unwound along the axis of the gear or worm.
2. The axial advance of a helix for one revolution (360°) as in the threads of cylindrical worms and the teeth of helical gears.

Lead Tolerance:
The allowable lead variation.

Lead Variation:
The difference between the measured lead trace and the specified (or theoretical) lead trace measured normal to the specified (or theoretical) lead trace.