Clearance \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.

Source: Webster's Revised Unabridged Dictionary (1913)