In logic, a tautology is a formula which is true in every possible interpretation. A formula is satisfiable if it is true under at least one interpretation, and thus a tautology is a formula whose negation is unsatisfiable. Unsatisfiable statements, both through negation and affirmation, are known formally as contradictions. A formula that is neither a tautology nor a contradiction is said to be logically contingent. Such a formula can be made either true or false based on the values assigned to its propositional variables.
-- cf Wikipedia: http://en.wikipedia.org/wiki/Tautology_%28logic%29