Infiltration is defined as:

Infiltration refers to the process by which outdoor air enters a building through various openings, such as gaps and cracks around windows and doors. This can occur due to differences in air pressure inside and outside the building, mainly caused by wind and temperature changes. When air leaks in around these structures, it can contribute to heating and cooling inefficiencies, impacting the overall indoor air quality and energy consumption of the HVAC system.

The other options describe different concepts related to indoor air but do not accurately define infiltration. Circulating air through an HVAC system relates to conditioned air that is being actively moved throughout a space, whereas conditioned air is air that has been heated or cooled to provide comfort within the home. Lastly, air that is filtered by an air purifier pertains to the purification of indoor air rather than its movement or entry from the outside. Understanding infiltration is crucial as it plays a significant role in energy efficiency, comfort levels, and indoor air quality in heating and cooling systems.