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What is the definition of no-load current of a transformer?

The RMS value of the current flowing through a line terminal of a winding when rated voltage (tapping voltage) is applied at rated frequency, meantime the other winding or windings being open circuited.

NOTE 1. For a three-phase transformer, the value is the arithmetic mean of the values of current in the three phases.

NOTE 2. The no-load current of a winding is often expressed as a percentage of the rated current of that winding.
For a multi-winding transformer this percentage is referred to the winding with the highest rated power.