Hendiadys (pronounced /hɛnˈdaɪ.ədɨs/, a Latinized form of the Greek phrase ἓν διὰ δυοῖν, hèn dià duoîn, "one through two") is a figure of speech used for emphasis, and is defined as the substitution of a conjunction for a subordination. The basic idea is to use two words linked by a conjunction (e.g. and) to express a single, complex idea.
Examples - sound and fury (from Macbeth) versus the furious sound. The Kingdom, the power and the glory (from the Lord's Prayer) instead of a glorious, powerful kingdom
This is to be distinguished from a tautology, which is an unnecessary or unessential (and sometimes unintentional) repetition of meaning, using different and dissimilar words that effectively say the same thing.
Examples - free gift, a short summary or a new innovation.