What is the definition of load loss of a transformer?

The absorbed active power at rated frequency and reference temperature associated with a pair of windings when rated current (tapping current) is flowing through the line terminals of one of the windings, and the terminals of the other winding are short circuited.

Further windings, if existing, are open-circuited.

NOTE 1. For a two-winding transformer there is only one winding combination and one value of load loss. For a multi-winding transformer there are several values of load loss corresponding to the different two-winding combinations.