Auto-antonym

An auto-antonym or autantonym, also called a contronym, contranym or Janus word, is a word with multiple meanings (senses) of which one is the reverse of another. For example, the word "cleave" can mean "to cut apart" or "to bind together". This phenomenon is called enantiosemy, enantionymy ("enantio-" means "opposite"), antilogy or autantonymy. An enantiosemic term is necessarily polysemic.
Nomenclature.
The terms "autantonym" and "contronym" were coined by Joseph Twadell Shipley in 1960 and Jack Herring in 1962, respectively. An auto-antonym is alternatively called an antagonym, Janus word (after the Roman god Janus, who is usually depicted with two faces), enantiodrome, enantionym, self-antonym, antilogy, or addad (Arabic, singular didd).
Linguistic mechanisms.
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. 
Other contronyms are a form of polysemy, but where a single word acquires different and ultimately opposite definitions. For example, "sanction"—"permit" or "penalize"; "bolt" (originally from crossbows)—"leave quickly" or "fix/immobilize"; "fast"—"moving rapidly" or "unmoving". Some English examples result from nouns being verbed in the patterns of "add <noun> to" and "remove <noun> from"; e.g. "dust", "seed", "stone". Denotations and connotations can drift or branch over centuries. 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". Negative words such as "bad" and "sick" sometimes acquire ironic senses referring to traits that are impressive and admired, if not necessarily positive ("that outfit is bad as hell"; "lyrics full of sick burns").
Some contronyms result from differences in 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 (where British English would have "shelve", which in this sense has an identical meaning in American English). To "", in Australian English, is to loudly demonstrate support, while in British English it is to express disapproval and contempt.
Some words contain simultaneous opposing or competing meanings in the same context, rather than alternative meanings in different contexts; examples include blend words such as "coopetition" (meaning a murky blend of cooperation and competition), "frenemy" (meaning a murky blend of friend and enemy), "glocalization", etc. These are not usually classed as contronyms, but they share the theme of containing opposing meanings.
Auto-antonyms exist in many languages, as the following examples show.
In Latin, "sacer" has the double meaning "sacred, holy" and "accursed, infamous". Greek δημιουργός gave Latin its "demiurgus", from which English got its "demiurge", which can refer either to God as the creator or to the devil, depending on philosophical context.
In many languages, a word stem associated with a single event may treat the action of that event as unitary, so it can refer to any of the doings or persons on either side of the transaction, that is, to the action of either the subject or the object, or to either the person who does something or the person to whom (or for whom) it is done. Other cues nail down the aspects of subject versus object. Thus there is a simple logic involved, despite that discussions of such words sometimes fixate on a superficial appearance of illogic (that is, "how can one word mean both?!") such as in words for "borrow" and "lend"; see #Examples below.
Examples.
Other languages.
Seeming auto-antonyms can occur from translation. In Hawaiian, for example, "aloha" is translated both as "hello" and as "goodbye", but the essential meaning of the word is "love", whether used as a greeting or farewell. The Italian greeting "ciao" is translated as "hello" or "goodbye" depending on the context; however, the original meaning was "(I'm your) slave".