Available Seat Kilometres (ASK) is a measure of an airline's passenger carrying capacity.
The definition is:
seats available ×distance flown
By calculating the ASK for each plane in an airline’s fleet, the ASK of the fleet can be determined.
A seat-kilometre is available when a seat that is available for carrying a passenger is flown one kilometre.
The ASK for an airline is, in effect, the maximum number of kilometers it could fly passengers if ALL the seats are occupied.