Jónsson terms

Summary

Jónsson terms are a finite chain of ternary terms of a variety that connect the first projection to the third by identities. By Jónsson's theorem, a variety has Jónsson terms if and only if it is a congruence-distributive variety. They give a Mal'cev condition for congruence distributivity, so this property of all algebras of a variety reduces to the existence of finitely many terms.