The concept of unification is one of the main ideas behind logic programming, best known through the language Prolog. It represents the mechanism of binding the contents of variables and can be viewed as a kind of one-time assignment .A unification of two terms is a join (in the lattice sense) with respect to a specialization order. Unification is a binary operation whose purpose is to attempt to make its two operands the same.The process is similar to pattern matching, in fact, unification is often called two-way matching in that none of the operands is considered a pattern with respect to the other.