PDB Server - Keyword Definition
Illegal positionThe position cannot be achieved by a legal sequence of moves from the game array. This is generally regarded as a defect. But where an interesting effect cannot be achieved without illegality, the defect may be tolerated.
Retrograde analysis does not apply to illegal positions, except to prove their illegality.
An illegal position does not imply that the composition is either fairy or a joke.
In some problems with the stipulation "Add pieces", the starting diagram is illegal, but that's because it awaits completion through adding the missing pieces when a legal position will be formed.