 Before stating Blok and Pigozzi’s definition we need to introduce a notational convention. Given an algebra \(\bA\) and a set of equations \(\iEq\) in one variable, we denote by \(\tEq(\bA)\) the set of elements of \(\bA\) that satisfy all the equations in \(\iEq\). Then a logic \(\bL\) is said to have an algebraic semantics if there is a class of algebras \(\bK\) and a set of equations \(\iEq\) in one variable such that