A class of algebras is congruence distributive if each of its members is congruence distributive.
Congruence distributivity has many structural consequences. The most striking one is perhaps Jónsson's Lemma [Bjarni Jónsson, Algebras whose congruence lattices are distributive, Math. Scand. 21 (1967) 110--121 (1968) MRreview] which implies that a finitely generated CD variety is residually finite.
Equationally definable principal congruences