blend of contra- and -onym, on the pattern of synonym and antonym.
It is a word with multiple meanings, one of which is defined as the reverse of one of its other meanings. This phenomenon is called enantiosemy, enantionymy or antilogy.
An auto-antonym (sometimes spelled autantonym), or contronym (also spelled contranym), is a word with a homograph (another word of the same spelling) which is also an antonym (a word with the opposite meaning). An auto-antonym is alternatively called an antagonym, Janus word (after the Roman god), enantiodrome, self-antonym, antilogy, or addad (Arabic, singular didd). It is a word with multiple meanings, one of which is defined as the reverse of one of its other meanings. This phenomenon is called enantiosemy, enantionymy or antilogy.
The terms "autantonym" and "contronym" were coined by Joseph T. Shipley in 1960 and Jack Herring in 1962, respectively. Some pairs of contronyms are true homographs, i.e., distinct words with different etymology which happen to have the same form. For instance cleave "separate" is from Old English clēofan, while cleave "adhere" is from Old English clifian, which was pronounced differently. This is related to false friends, but false friends do not necessarily contradict.
Other contronyms are a form of polysemy, but where a single word acquires different and ultimately opposite senses. For instance quite, which meant "clear" or "free" in Middle English, can mean "slightly" (quite nice) or "completely" (quite beautiful). Other examples include sanction — "permit" or "penalize"; bolt (originally from crossbows) — "leave quickly" or "fix"; fast — "moving rapidly" or "unmoving". Many English examples result from nouns being verbed into distinct senses "add <noun> to" and "remove <noun> from"; e.g. dust, seed, stone.
Some contronyms result from differences in national varieties of English. For example, to table a bill means "to put it up for debate" in British English, while it means "to remove it from debate" in American English.
Often, one sense is more obscure or archaic, increasing the danger of misinterpretation when it does occur; for instance, the King James Bible often uses "let" in the sense of "forbid", a meaning which is now obsolete, except in the legal phrase "without let or hindrance" and in tennis, squash and table tennis.
An apocryphal story relates how Charles II (or sometimes Queen Anne) described St Paul's Cathedral (using contemporaneous English) as "awful, pompous, and artificial," with the meaning (rendered in modern English) of "awe-inspiring, majestic, and ingeniously designed."
In addition, various neologisms or other such words contain simultaneous opposing meanings in the same context, rather than alternative meanings depending on context (e.g. coopetition).
Auto-antonyms also exist in other languages. For example, in Latin sacer has the double meaning "sacred, holy" and "accursed, infamous",
Sometimes an apparent opposition of senses comes from presuming the point of view of a different language. In Hawaiian, for example, aloha is translated both as “hello” and as “goodbye”, but the essential meaning of the word is “love”, making it appropriate as both greeting and farewell. The meaning is in fact the same; it is only the occasion that is different. The Italian greeting ciao is translated as "hello" or "goodbye" depending on the context; however the original meaning was “(I'm your) slave” and so it's appropriate as both greeting and farewell. Latin altus can be translated "high" or "deep" in English, but in Latin had the single meaning "large in the vertical dimension". The difference in English between "high" and "deep" is determined by the speaker's awareness of their relationship to some perceived norm. A mountain is "high" because it is well above sea level, and the ocean is "deep" because it plunges well below it. Both, however, were altus in Latin.