Wikipedia defines tautology well
In grammar, a tautology (from Greek tauto, “the same” and logos, “word/idea”) is an unnecessary repetition of meaning, using more than one word effectively to say the same thing (often originally from different languages). It is considered a fault of style and was defined by A Dictionary of Modern English Usage (Fowler) as “saying the same thing twice”, when it is not apparently necessary to repeat the entire meaning of a phrase. “Fatal murder” is an example of a tautology. If a part of the meaning is repeated in such a way that it appears as unintentional, or clumsy, then it may be described as tautological. On the other hand, a repetition of meaning that improves the style of a piece of speech or writing is not necessarily tautological.
Here are some example sentences:
- He went there personally.
- The evening sunset was golden.
- My first priority is to quit smoking.
- There is a lot of frozen ice in winter.
- I heard the story with my own ears.
- Scientists like making predictions about the future.
- The hospital is in close proximity to the school.
- The Sahara is a very dry desert.
- In my opinion, I think she is right.
- Having an eye test is a necessary requirement for passing the driving test.
- She is a dark-haired brunette.
- He wrote an autobiography of his own life.