dedendum

Century Dictionary and Cyclopedia

1. n. That part of the tooth of a cog-wheel or gear which is inside the pitch-circle and is intercepted between the pitch-line and the circle which limits all the roots of the teeth and the spaces between them. The addendum of the teeth of the meshing-wheel enters into this space.

Wiktionary

1. n. The difference between the radius of the pitch circle of a gear and its root circle