‘Cooling capacity’ is the measure of an air conditioning system’s ability to remove heat from a room, thus making the room ‘cooler’.
The reverse of this gives you ‘heating capacity’ (i.e. an air conditioner's ability to remove heat from outside and transferring this into the room).
Capacity in these instances is always measured using the rated conditions specified in the official document: Australian/New Zealand Standard: Performance of electrical appliances – Air conditioners and heat pumps (also known as ‘AS/NZS 3823’).
Generally the unit used for this measurement is specified in Kilowatts (kW).