A euphemism /ˈjuːfəˌmɪzəm/ is a generally innocuous word or expression used in place of one that may be found offensive or suggest something unpleasant. Some euphemisms are intended to amuse, while others use bland, inoffensive terms for things the user wishes to downplay. Euphemisms are used to refer to taboo topics (such as disability, sex, excretion, or death) in a polite way, or to mask profanity.
The word comes from Greek εὐφημία (euphemia) "the use of words of good omen", which is a compound of eû (εὖ) "good, well" and phḗmē (φήμη) "prophetic speech; rumour, talk". The eupheme is the opposite of the blaspheme "evil-speaking". The term euphemism itself was used as a euphemism by the ancient Greeks, meaning "to keep a holy silence" (speaking well by not speaking at all).
Reasons for using euphemisms vary by context and intent. A common usage of euphemisms serves the avoidance of issues with perceived negative connotations or subjects of personal embarrassment. In rarer cases, euphemisms are employed to downplay the gravity of large-scale injustices, war crimes, or other occurrences that warrant a pattern of avoidance in the official language. For instance, one reason for the comparative scarcity of written evidence documenting the exterminations at Auschwitz (at least given the scale) is "directives for the extermination process obscured in bureaucratic euphemisms".
The euphemism affirmative action meaning a preference for minorities or the historically disadvantaged usually in employment or academic admissions (also called reverse discrimination, or in the UK positive discrimination) is an example of a well-intentioned circumlocution to disguise intentional bias that might be legally prohibited, or otherwise unpalatable. 
A euphemism such as enhanced interrogation for torture may be less benign: columnist David Brooks called the euphemisms for torture at Abu Ghraib, Guantánamo and elsewhere an effort to "dull the moral sensibility".
Phonetic euphemism is used to replace profanities, giving them the intensity of a mere interjection.
The use of a term with a softer connotation, though it shares the same meaning. For instance, screwed up is a euphemism for fucked up; hook-up and laid are euphemisms for sexual intercourse.
There is some disagreement over whether certain terms are or are not euphemisms. For example, sometimes the phrase visually impaired is labeled as a politically correct euphemism for blind or a blind person. However, visual impairment can be a broader term, including, for example, people who have partial sight in one eye, those with uncorrectable mild to moderate poor vision, or even those who wear glasses, groups that would be excluded by the word blind.
Expressions or words from a foreign language may be imported for use as a replacement for an offensive word. For example, the French word enceinte was sometimes used instead of the English word pregnant. This practice of word substitution became so frequent that the expression "pardon my French " was adopted in attempts to excuse the use of profanity.
Euphemisms may be formed in a number of ways. Periphrasis, or circumlocution, is one of the most common: to "speak around" a given word, implying it without saying it. Over time, circumlocutions become recognized as established euphemisms for particular words or ideas.
To alter the pronunciation or spelling of a taboo word (such as a swear word) to form a euphemism is known as taboo deformation, or a minced oath. In American English, words that are unacceptable on television, such as fuck, may be represented by deformations such as freak, even in children's cartoons. Some examples of rhyming slang may serve the same purpose: to call a person a berk sounds less offensive than to call a person a cunt, though berk is short for Berkeley Hunt, which rhymes with cunt.
Bureaucracies frequently spawn euphemisms of a more deliberate nature, doublespeak expressions. For example, in the past, the US military used the term "sunshine units" for contamination by radioactive isotopes. An effective death sentence in the Soviet Union during the Great Purge often used the clause "imprisonment without right to correspondence": the person sentenced never had a chance to correspond with anyone because soon after imprisonment he would be shot. As early as 1939, Nazi official Reinhard Heydrich used the term Sonderbehandlung ("special treatment") to mean summary execution (most likely by hanging) of persons viewed as "disciplinary problems" by the Nazis even before commencing the systematic extermination of the Jews. Heinrich Himmler, aware that the word had come to be known to mean murder, replaced that euphemism with one in which Jews would be "guided" (to their deaths) through the slave-labor and extermination camps after having been "evacuated" to their doom. Such was part of the superficially innocuous formulation Endlösung der Judenfrage (the "Final Solution to the Jewish Question"), which became infamous to the entire world during the Nuremberg Trials.
A euphemism may itself devolve into a taboo word, through the linguistic process known as semantic change (specifically pejoration) described by W.V.O. Quine, and more recently dubbed the "euphemism treadmill" by Harvard professor Steven Pinker. For instance, toilet is an 18th-century euphemism, replacing the older euphemism house-of-office, which in turn replaced the even older euphemisms privy-house and bog-house. In the 20th century, where the words lavatory or toilet were deemed inappropriate (e.g. in the United States), they were sometimes replaced with bathroom or water closet, which in turn became restroom, W.C., or washroom.
Doublespeak is a term sometimes used for deliberate euphemistic misuse of incorrect words to disguise unacceptable meaning, as in a "Ministry of Peace" which wages war, and a "Ministry of Love" which imprisons and tortures. It is a portmanteau of the terms Newspeak and doublethink, which originate from George Orwell's novel 1984.
The word euphemism itself can be used as a euphemism. In the animated TV special Halloween Is Grinch Night (see Dr. Seuss), a child asks to go to the euphemism, where euphemism is being used as a euphemism for outhouse. This euphemistic use of euphemism also occurred in the play Who's Afraid of Virginia Woolf? where a character requests, "Martha, will you show her where we keep the, uh, euphemism?"
In Wes Anderson's film Fantastic Mr. Fox, the replacement of swear words by the word cuss became a humorous motif throughout the film.
In Tom Hanks's web series Electric City, the use of profanity has been censored by the word expletive. "[Expletive deleted]" entered public discourse after its notorious use in censoring transcripts of the Watergate tapes.
In Isaac Asimov's Foundation series, the curses of the scientist Ebling Mis have all been replaced with the word unprintable. In fact, there is only one case of his curses being referred to as such, leading some readers to mistakenly assume that the euphemism is Ebling's, rather than Asimov's. The same word has also been used in his short story "Flies".
George Carlin has stated in audio books and his stand up shows that euphemisms soften everyday language and take the life out of it. The use of euphemisms, as he has stated, is so that white Americans, especially those he calls whiny, narcissistic baby boomers, will not have to face the toughness of life.
Uglier even than human-rights abuses and more obscure even than comfort station, affirmative action is a euphemism with little to be said for it.
Chapter 4: Affirmative Action Diversity: A Euphemism for Preferences, Quotas, and Set-asides
In modern times, various social and political movements have introduced euphemisms, from affirmative action to political correctness to international conflicts, which are linguistically and culturally driven.
[T]he report . . . cuts through the ocean of euphemism, the EITs, enhanced interrogation techniques, and all that. It gets to straight language. Torture — it's obviously torture. . . . the metaphor and the euphemism is designed to dull the moral sensibility.
Enhanced interrogation has always been a kind of handy euphemism (for torture)
The Honest Jakes or Privy has graduated via Offices to the final horror of Toilet.