A mechanism -- using a combination of round/circular and flat "gears" -- designed to convert rotational motion into linear motion. In its operation a "rack-and-pinion" mechanism engages teeth on a flat bar of the rack -- with corresponding teeth on a round "gear", the pinion. Rotating the pinion causes the rack to move laterally.
In woodworking, an example of a rack and pinion arrangement is commonly found in the floor drill press, where -- the drill's operator can adjust appropriately the distance from the drill's cutting bit to the workpiece to be drilled -- this mechanism allows raising and/or lowering of the drill press's "table".