Vacuum is a relative condition, normally referring to a lower pressure than atmospheric in any given confined volume.

A vacuum pump is defined as a pump that removes a gas from a sealed volume in order to leave behind a partial vacuum. A vacuum pump can be visualised as a compressor running with the inlet attached to the vacuum system and the outlet attached to the exhaust.

Pumps can be broadly grouped into four main categories

Positive displacement pumps
Non-positive displacement pumps
Molecular pumps