A vacuum is defined colloquially as the state encountered in a room at pressures below atmospheric pressure. These pressures can be generated by gases or vapors that are evenly distributed over the room.

The standard definition of vacuum is “the state of a gas at which its pressure in a vessel and therefore its particle density is lower than that of the ambient surrounding atmosphere or in which the pressure of the gas is lower than 300 mbar, i. e. lower than the pressure of the atmosphere on the Earth’s surface.“

In a range of technical applications, the pressure is not indicated as absolute but as relative to atmospheric pressure. The pressure range below atmospheric pressure is indicated as a negative number or a percentage. Examples of this are manometers, pressure reducers on gas cylinders or uses for vacuum lifting gear or vacuum transport systems.
Different types of vacuum pumps are used on Earth to generate a vacuum. An overview of the working ranges of the most important types of vacuum pump and vacuum instruments is given in Figure 1.1: Overview of vacuum.

Figure 1: Overview of vacuum