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A laboratory vacuum is a region with substantially reduced gas pressure, rather than an experimentally perfect absence of matter. Lowering the number density increases the mean free path, allowing a molecular beam to travel with fewer collisions. In molecular flow, the mean free path exceeds the apparatus dimensions, so wall encounters dominate.

A vacuum pump competes with leaks and outgassing. For an isothermal vessel of volume $V$, constant gas load $Q$ in pressure-volume per unit time, and effective pumping speed $S$, gas balance givesThis ideal model predicts a pump-down time constant $V/S$ and a limiting pressure $Q/S$. A restricted connecting pipe reduces the effective pumping speed; a larger pump alone need not fix it. Measuring the pressure transient tests the model, while deviations can reveal changing outgassing or pumping speed.^[ref]

A vacuum pump lowers gas pressure by removing gas from a vessel or trapping it so that it no longer contributes to the free gas population. Mechanical displacement, momentum transfer and capture are distinct operating mechanisms.

Its relevant performance includes pumping speed, attainable pressure and gas dependence. The pressure achieved in an apparatus also depends on connecting vacuum conductance, leaks and outgassing; the pump's advertised limit is therefore not a complete system specification.^[ref]