Air change is the number of times the total air volume of a space is replaced by fresh air from an HVAC system in a one hour timeframe.

Example:

A 2,000ft³ space with an HVAC air flow of 10,000ft³/hr (cfh) would result in an air change rate of 5 times per hour.

Here is a chart with typical air change rates for different spaces.

Image Source: www.engineeringtoolbox.com