-  An abbreviation for air conditioning or air conditioner. An abbreviation for "across corners" which indicates the distance on a nut (for instance) from one corner to the opposite corner rather than the distance from one flat surface (A/F) to the opposite (which would be the size of wrench needed to install or remove the nut). The purpose of the A/C dimension is to know how large a hole might be needed to insert a recessed nut.
Dictionary of automotive terms. 2015.