In grammar, a tautology (from Greek tauto, "the same" and logos, "word"/"idea") is a statement that is tautological, or 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.
Intentional repetition of meaning intends to amplify or emphasize a particular, usually significant, fact about what is being discussed. For example, a gift is, by definition, free of charge; using the phrase "free gift" might emphasize that there are no hidden conditions or fine print, be it the expectation of money or reciprocation, or that the gift is being given by volition.
This is related to the rhetorical device of hendiadys, where one concept is expressed through the use of two, for example "goblets and gold" meaning wealth, or "this day and age" meaning the present time (meaning "now"). Superficially these expressions may seem tautological, but they are stylistically sound because the repeated meaning is just a way to emphasise the same idea.
The use of tautologies is, however, usually unintentional. They often hinder reader comprehension and undermine the writer's credibility. As Kallan explains, "Mental telepathy, planned conspiracies, and small dwarfs, for example, convey the possibility of physical telepathy, spontaneous conspiracies, and giant dwarfs." (Kallan)
Fowler offers that some tautologies derive from historic processes. One example of this is that when the Bible was translated into Anglo-Saxon, Norman French was still common among the aristocracy, so expressions like "save and except" were translated both for the commoners and the aristocrats; although in this case both "save" and "except" have a French or Latin origin.
Fowler makes a similar case for double negatives; in Old English they intensified the expression, did not negate it back to being a positive, and plenty of examples exist in writings before the eighteenth century, such as Shakespeare. In Modern French, for example, the "ne-pas" formation is essentially a double negative,as also is the Russian form, "Ya ne vizhu nichevo" (literally I don't see nothing). and in many other Western European latinate languages the same applies, with "ni" or "no", mutatis mutandis, emphasising instead of negating the initial negative. In common French, the "ne" is quite typically dropped, as it was believed to have been in Vulgar Latin.
Parallelism is not tautology, but rather a particular stylistic device. Much Old Testament poetry is based on parallelism: the "same thing" said twice, but in slightly different ways (Fowler puts it as pleonasm). However, modern biblical study emphasizes that there are subtle distinctions and developments between the two lines, such that they are usually not truly the "same thing." Parallelism can be found wherever there is poetry in the Bible: Psalms, the Books of the Prophets, and in other areas as well.