noun
1. : an air space with two airtight doors or openings for permitting entrance to or exit from a sealed-off space under pressure (as a ship's stokehole) or vacuum: as
a. : an intermediate chamber between the outer air and the working chamber of a pneumatic caisson
b. : the chamber in an electron microscope between outer air and inner vacuum
c. : the entrance chamber to a gas-proof shelter
2. : a stoppage of flow (as in a pumping device) caused by air being in a part where liquid ought to circulate — compare vapor lock