Tautology, n.
Etymology: [L. tautologia, Gr. tautologie.]
(Rhet.)
A repetition of the same meaning in different words; needless repetition of an idea in different words or phrases; a representation of anything as the cause, condition, or consequence of itself, as in the following lines: -- The dawn is overcast, the morning lowers, And heavily in clouds brings on the day. Addison.
Syn. -- Repetition. -- Tautology, Repetition. There may be frequent repetitions (as in legal instruments) which are warranted either by necessity or convenience; but tautology is always a fault, being a sameness of expression which adds nothing to the sense or the sound.
Shades of meaning
Fernald has no entry of its own for tautology; he lists it under circumlocution.
Where he writes about it:
Pleonasm is the expression of an idea already plainly implied; tautology is the restatement in other words of an idea already stated, or a useless repetition of a word or words. — from circumlocution
Pleonasm may add emphasis; tautology is always a fault. — from circumlocution
"I saw it with my eyes" is a pleonasm; "all the members agreed unanimously" is tautology. — from circumlocution