A tautology is a needless repetition of an idea, especially in words other than those of the immediate context, without imparting additional force or clearness, as in ‘Morning sunrise.’
In logic, a tautology is a compound propositional form all of whose instances are true, as A or not A. An instance of such a form, as ‘This candidate will win or will not win.’
In other words, tautology is unnecessary repetition. For example ‘They spoke in turn, one after the other’ is considered a tautology because ‘in turn’ and ‘one after the other’ mean the same thing.
“At a certain point talk about ‘essence’ and ‘oneness’ and the universal becomes more tautological than inquisitive.” – Christopher Hitchens