or "driving tooth"

The driving tooth presses on the driven tooth. The normal at the point of contact is the direction of this pressure, since the two surfaces can, and do, slide on each other in the tangent direction and cannot exert any force in this direction. The direction of the normal is called the line of pressure, which in this case passes through the center of the rod, and the point where the flank of the driver tooth contacts the rod. The behavior of this line as the gears rotate is of great importance in gearing, since it gives the instantaneous speed ratio of the gears. In the present case, when the gear flanks first come into contact on approach the driven gear is forced ahead faster than average. As the teeth mesh more deeply, the driven gear is moved more slowly than average. As the teeth recede, the speed of the driven gear is again urged more rapidly forward. Of course, the driven gear cannot respond without inertia, so the result is a periodic strain that causes torsional vibrations in the shafts, and extra stress on the gear teeth.

http://mysite.du.edu/~jcalvert/tech/oldgears.htm