A device that reduces the pressure of a gas (usually air) in a container. When gas in a closed container is lowered from atmospheric pressure, the operation constitutes an increase in vacuum in this container. The unit used for vacuum is a millimeter (mm) column of mercury (Hg). Another term for the mmHg unit is the torr. These terms are used interchangeably. The torr is named in honor of E. Torricelli, a pioneer in vacuum technology. The various vacuum regions are classified asfollows:

• Low vacuum, 760 (atmospheric pressure) to 25   mmHg
• Medium vacuum, 25 to 10⁻³ mmHg
• High vacuum, 10⁻³ to 10⁻⁶ mmHg
• Very high vacuum, 10⁻⁶ to 10⁻⁹ mmHg
• Ultrahigh vacuum, 10⁻¹⁰ mmHg and beyond

A vacuum as high as 10⁻¹⁵ mmHg or one-billionth of a billionth of an atmosphere has been reached.