From Online Dictionary of Crystallography
Défaut d'empilement (Fr). Stapelfehler</font) (Ge). Difetto d'impilamento (It). 積層欠陥 (Ja). fallo de apilamiento (Sp)
A stacking fault is a type of planar defect that occurs in modular crystal structures, and especially (but not exclusively) in layer structures. In a regular, periodic sequence of of modules, a stacking fault corresponds to an out-of-sequence module, i.e. a module that differs in its orientation / position from what would be expected in a perfectly periodic sequence.
Stacking faults can arise during crystal growth or from plastic deformation. They are in a higher energy state, quantified by the stacking-fault energy, which corresponds to the formation enthalpy per unit area.