Definition of tautology – the saying of the same thing twice over in different words, generally considered to be a fault of style. noun, plural tautologies. needless repetition of an idea, especially in words other than those of the immediate context, without imparting additional force or clearness, as in widow woman.. … noun plural -gies. Quotes the learning and the use in particular. Having taken in view, we tend to used this in common language unconsciously in particular.