The term returning bishop refers to a bishop that went back to its original square after allowing own king to castle in the same direction. The typical example would be something like that: 1.e4 c5 2.Nf3 Nc6 3.Bb5 d6 4.0-0 Bd7 5. c3 Nf6 6.Re1 a6 7.Bf1.
Simplistic piece-square evaluation would treat such a bishop as undeveloped. Yet it is a good defender and, more importantly, "developing" it would waste another tempo. For that reasons it might be beneficial to assign a bonus erasing the development penalty for such a bishop.
The idea has been proposed (or just reinvented?) by Pawel Koziol.