The angle δ in the swing equation of a synchronous

1. angle between stator voltage and current.
2. angular displacement of the rotor with respect to the stator.
3. angular displacement of the stator mmf with respect to a synchronously rotating axis.
4. angular displacement of an axis fixed to the rotor with respect to a synchronously rotating axis.

In swing equation δ is the angular displacement of an axis fixed to the rotor with respect to a synchronously rotating axis.