Clearance /(klēr"ans)/

Clear·ance

Clearance

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.
  4. 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. (Mach.)

Phrases & Compounds
Clearance space
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.