rack and pinion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A device for the conversion of rotary and linear motion, consisting of a pinion and a mated rack.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A pair of gears, consisting of a circular pinion that engages with the teeth of a flat bar, that converts rotational into linear motion; used in the steering mechanism of cars, and in some railways

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a wheel gear (the pinion) meshes with a toothed rack; converts rotary to reciprocating motion (and vice versa)