In practical engineering terms, a vacuum or partial vacuum describes a space containing a gas where the pressure of the gas is lower than atmospheric pressure.
The two easiest ways to create a partial vacuum are: 1) expand the volume of a sealed gas-filled container; or 2) reduce the mass of the gas inside a fixed size container.
Volume expansion:
When a piston is pulled downwards in a cylinder by the rotation of the crankshaft, the volume inside the cylinder is expanded, reducing the internal pressure of the gases in the cylinder, and creating a partial vacuum. When the intake valve opens, the higher atmospheric pressure outside the cylinder pushes air-fuel mixture into the cylinder until the pressure has again been equalized.
Mass reduction:
A vacuum pump works by repeatedly creating a small area of low pressure inside a sealed container, allowing the gas in the container to fill that area, then pushing that volume of gas through a one way valve and out of the container. Each successive evacuation of a small amount of gas reduces the mass of gas in the container and therefore the pressure of that gas.
Pressure is defined as a measure of force per unit area, and is most commonly measured in pascals (Pa) in the SI system, which is equal to one Newton of force per square meter, or in pounds per square inch (psi) in the Imperial system.
