When a chamber has no leaks, no gas deliberately flowing into it, and has been pumped for several days, the pressure reaches an equilibrium value called the base pressure. In truth, because the pressure approaches equilibrium asymptotically and outgassing rate undergoes exponential decay, even a long time under vacuum, the chamber, theoretically, will never quite reach a stable pressure. But variations in vacuum gauge calibration, room temperature, pumping speed, back streaming from the pump, etc., mask or counter any real pressure reduction and the chamber appears to have reached a steady state.
Often what happens is: the operator pumps the chamber for a few hours, grows tired of waiting, and claims the chamber is at base pressure.
This is not necessarily wrong. After all, if the pressure falls from 5 × 10-7 Torr to 4 × 10-7 Torr by waiting another ten hours, is all that much gained? Perhaps it doesn't comform to formal definition, but in a sense the base pressure is reached whenever the operator says it is and starts using the chamber.