Characteristic Formulas 50 Years Later (An Algebraic Account)
  The Jankov (characteristic) formulas were introduced by V.Jankov fifty tears ago in 1963. Nowadays the Jankov (or frame) formulas are used in virtually every branch of propositional logic: intermediate, modal, fuzzy, relevant, many-valued, etc. All these different logics have one thing in common: in one form or the other, they admit the deduction theorem. From a standpoint of algebraic logic it means that their corresponding varieties have a ternary deductive (TD) term. It is natural to extend the notion of characteristic formula to such varieties and, thus, apply this notion to an even broader class of logics, namely, to the logics which algebraic semantic is a variety with a TD term. 
Alex Citkin
math.LO