Two useful grammatical terms used when discussing certain words and phrases refer to tautology and autology. While the terms sound similar, they have very different meanings.
Tautology (sometimes also called circumlocution) is used to describe a phrase where the same thing is said more than once, even though it does not help to clarify the overall meaning of the phrase or add any further information. For example, the common phrase “free gift” is a tautology, since a gift is free by its very nature. Further often-seen examples of tautological phrases are “added bonus”, “planning ahead”, and “first introduced”. There are many, many more of these.
Tautology often occurs when we use words or phrases borrowed from other languages and integrate them into English phrases, for example “the hoi polloi” (meaning “the masses”, or “the common people”). This is from Ancient Greek (ὁι πολλοι), and is literally translated as “the many” (hoi = the, polloi = many men). Since hoi already means “the”, when we say “the hoi polloi“, we’re repeating “the”.
More familiar examples of this kind of mixed language tautology are often found in the names of places and animals, including “Sahara Desert” (sahara is Arabic for “desert” (صَحراء), “ṣaḥrā´”), so it means “desert desert”), “Koi carp” (koi is Japanese for “carp” (鯉), so it means “carp carp”), and the “Rio Grande river” (rio grande is Spanish for “big river”, so it means “big river river”).
Autology, despite being only one letter removed from tautology, is very different. Autology (from the Greek autos (αủτος), meaning “self”, and logos (λογος), meaning “word”) describes a word that expressing a property that the word has itself. For example, the word “short” is fairly short, the word “common” is common, and the word “multisyllabic” has many syllables. Autological words aren’t seen particularly often, but can be quite interesting to study – you can find a list of them on Wiktionary.com.
This isn’t to say that tautology and autology can never be seen together, however – you may remember that a while ago I wrote about RAS syndrome, “Repetitive Acronym Syndrome syndrome” – which is in itself both an example of itself and a tautology!