Gnu and Leeniea refer to the logical kind, which is different.
However, I don't think gnu's example is a logical tautology either. Ordinarily a tautology is a statement of fact(not, as in Gnu's example, a conditional statement), the meaning of whose terms alone make it logically irrefutable.
Something may be true whether anyone can prove it or not. Is "Forbidden Planet" the hottest movie rental somewhere in outer space? I don't think so, but I can't prove it or disprove it. It might be. Does Zeus run the cosmos? Unlikely but not disprovable.