Clear"ance (-ans), n. 1. The act of clearing; as, to make a thorough clearance.

2. A certificate that a ship or vessel has been cleared at the customhouse; permission to sail.

Every ship was subject to seizure for want of stamped clearances.
Durke

3. Clear or net profit. Trollope.

4. (Mach.) The distance by which one object clears another, as the distance between the piston and cylinder head at the end of a stroke in a steam engine, or the least distance between the point of a cogwheel tooth and the bottom of a space between teeth of a wheel with which it engages.

Clearance space(Steam engine), the space inclosed in one end of the cylinder, between the valve or valves and the piston, at the beginning of a stroke; waste room. It includes the space caused by the piston's clearance and the space in ports, passageways, etc. Its volume is often expressed as a certain proportion of the volume swept by the piston in a single stroke.