In order that the teeth of gears and pinions may run together smoothly and with a constant relative velocity, it is necessary that their working face shall be formed of certain curves called odontoids. The essential property of these curves is that when two teeth are in contact the common normal to the tooth curves at their point of contact must pass through the pitch point, or point of contact of the two pitch-circles. Two such curves are in common use, viz. the cycloid and the involute.

Kent's Mechanical Engineering Handbook, Wiley, Tenth Edition 1923

in vo lute (in've loot') --n Math a)the curve traced by any point of a taut string when it is wound upon or unwound from a fixed curve on the same plane with it.

Webster's New World Dictionary , Collins, Second Edition, 1980