Acronym and initialism
Of the two words, acronym is the much more frequently used and known, and many speakers and writers refer to all abbreviations formed from initial letters as acronyms. However, many others differentiate between acronyms and initialisms. An acronym is a pronounceable word formed from the initial letter or letters of the constituent words, such as NATO. An initialism is an abbreviation pronounced as the names of the individual letters, and is formed only from the initial letter of constituent words, such as HTML. This distinction is supported by many dictionary definitions, but not by all.
Acronyms and initialisms are a relatively new linguistic phenomenon, having only become popular during the 20th century. As literacy rates rose, the practice of referring to words by their first letters became increasingly convenient. The first recorded use of the word initialism in the Oxford English Dictionary (OED) is in 1899, and the first for acronym is in 1943. The word acronym comes from Greek: ακρον, akron, "limb" + ονομα, onoma, "name").
Nonetheless, earlier examples of acronyms exist. The early Christians in Rome used a fish as a symbol for Jesus in part because of an acronym — fish in Greek is ΙΧΘΥΣ (ichthus), which was said to stand for Ιησους Χριστος Θεου Υιος Σωτηρ (Iesous CHristos THeou Uios Soter: Jesus Christ, Son of God, Savior). Evidence of this interpretation dates from the second and third centuries and is preserved in the catacombs of Rome.
Acronyms and initialisms often occur in jargon or as names of organizations because they often serve as abbreviations of long terms that are frequently referenced, so a shortened form is desirable. Militaries and government agencies frequently employ acronyms and initialisms, perhaps most famously the US Government and the so-called alphabet agencies of Roosevelt's New Deal. Cynics have quipped that acronyms are used to obfuscate.
Abbreviations have been traditionally written using a full stop/period to mark the part that was deleted. In the case of most acronyms and initialisms, each letter is its own abbreviation, and in theory should get its own period. This usage is becoming less common as the presence of all capital letters is sufficient to indicate the word is an abbreviation; nevertheless some influential style guides still insist on the many-periods treatment, such as the one used by the New York Times (which recommends periods after unpronouncible acronyms such as "K.G.B." but not pronouncible ones, such as "NATO"  (http://www.nytimes.com/2004/02/07/opinion/07KRIS.html?ex=1391490000&en=f887afd296d59e2f&ei=5007&partner=GOOGLE)), but others, such as at the BBC, no longer require this.
Some acronyms undergo assimilation into ordinary words: often they are then written in lower case, and eventually it is widely forgotten that the word was derived from the initials of others: scuba and laser, for instance. The term anacronym has been coined as a portmanteau of the words anachronism and acronym to describe acronyms whose original meaning is not known to most speakers.
While typically abbreviations exclude the initials of short function words (such as "and", "or", "of", or "to"), they are sometimes included in acronyms to make them pronounceable.
The traditional style of pluralizing single letters with "'s" ("there are two Q's in that word") was naturally extended to acronyms when they were commonly written with periods, and is still preferred by some people, especially when the acronym is pronounced as separate letters. However, today it is more usual to inflect them like ordinary words; thus the usual plural of "CD" is "CDs", with "CD's" being reserved for the possessive.
In some cases, an acronym or initialism has been turned into a name. The letters making up the name of the SAT college entrance test, for example, no longer officially stand for anything. This trend has been common with many companies hoping to retain their brand recognition while simultaneously moving away from what they saw as an outdated image: American Telephone and Telegraph is now simply AT&T, the company formerly named Kentucky Fried Chicken changed its name to "KFC" (though whether to avoid negative connotations of the word "fried", or regional associations with Kentucky, or to allow products other than chicken remains a subject of speculation); British Petroleum changed its name to "BP" to emphasize that it was no longer only an oil company (captured by the motto "beyond petroleum"); and Silicon Graphics, Incorporated changed its name to "SGI" to emphasize that it was no longer only a computer graphics company. Initialisms may have advantages in international markets; for example, some national affiliates of International Business Machines are legally incorporated as "IBM" (or, for example, "IBM Canada") to avoid translating the full name into local languages.
Sometimes, the initials are kept but the meaning is changed. SADD, for instance, originally Students Against Drunk Driving, changed the full form of its name to Students Against Destructive Decisions. YM originally stood for Young Miss, and later Young & Modern, but now stands for simply Your Magazine.
When initialisms are defined in print, especially in the case of industry-specific jargon, the words forming the abbreviation are often capitalized for clarity. While this would be perfectly acceptable for proper nouns like Kentucky Fried Chicken, some usage writers have argued that it is technically incorrect for other terms like storage area network. Correct or not, such usage is widespread in English publications.
Initialism originally referred to abbreviations formed from initials, without reference to pronunciation, but during the middle portion of the twentieth century, when acronyms and initialisms saw more use than ever before, the word acronym was coined for abbreviations which are pronounced as a word, like "NATO" or "AIDS". The term initialism is now typically taken to refer to abbreviations which are pronounced by sounding out the name of each constituent letter (e.g. HTML). Some have extended the term acronym in meaning to describe all abbreviations made from initial letters, regardless of pronunciation.
There is no agreement on what category to place abbreviations in which contain single letters but can otherwise be pronounced as a word, such as JPEG (Jay-Peg). These abbreviations are sometimes referred to as acronym-initialism hybrids, although they are by some grouped under the broad meaning of acronym.
The longest acronym, according to the 1965 edition of Acronyms, Initialisms and Abbreviations Dictionary, is ADCOMSUBORDCOMPHIBSPAC, a United States Navy term that stands for "Administrative Command, Amphibious Forces, Pacific Fleet Subordinate Command".
The world's longest initialism, according to the Guinness Book of World Records is NIIOMTPLABOPARMBETZHELBETRABSBOMONIMONKONOTDTEKHSTROMONT. The 56-letter initialism (54 in Cyrillic) is from the Concise Dictionary of Soviet Terminology and means "The laboratory for shuttering, reinforcement, concrete and ferroconcrete operations for composite-monolithic and monolithic constructions of the Department of the Technology of Building-assembly operations of the Scientific Research Institute of the Organization for building mechanization and technical aid of the Academy of Building and Architecture of the USSR".
During the 1960s trend for action-adventure spy thrillers, it was a common practice for fictional spy organizations or their nemesis to employ names that were acronyms. Sometimes these acronyms made sense but most of the time, they were words incongrously crammed together for the mere purpose of obtaining a catchy acronym, traditionally a heroic sounding one for the good guys and an appropriately menacing one for the bad guys. This has become one of the most commonly parodied clichés of the spy thriller genre. Some of the most popular were: