Systematic absences
From Online Dictionary of Crystallography
Absences systématiques (Fr). Extinciones sistemáticas (Sp). Assenze sistematiche, estinzioni sistematiche (It)
Definition
One speaks of systematic absences or extinctions when the structure factor is zero, due either to the centring of the lattice or to the presence of glide or screw symmetry elements. Conversely, the conditions for a reflection to exist and not to be systematically absent, are called reflection conditions.
See also
reflection conditions
Sections 2.2.13 and 12.3.5 of International Tables of Crystallography, Volume A