For guidelines on making and editing abbreviation articles on Wikipedia, see Wikipedia:Disambiguation and abbreviations.
An abbreviation (from Latin brevis "short") is a shortened form of a word or phrase. Usually, but not always, it consists of a letter or group of letters taken from the word or phrase. For example, the word "abbreviation" can itself be represented by the abbreviation "abbr." or "abbrev."

Types of abbreviations
Related article: Clipping (lexicography)
A syllabic abbreviation (SA) is an abbreviation formed from (usually) initial syllables of several words, such as Interpol for International police, but should be distinguished from portmanteaux. They are usually written in lower case, sometimes starting with a capital letter, and are always pronounced as words rather than letter by letter.

Syllabic abbreviation
Syllabic abbreviations are not widely used in English or French, but are common in certain languages, like German and Russian.
They prevailed in Germany under the Nazis and in the Soviet Union for naming the plethora of new bureaucratic organizations. For example, Gestapo stands for Geheime Staats-Polizei, or "secret state police". This has given syllabic abbreviations a negative connotation, even though they were used in Germany before the Nazis, such as Schupo for Schutzpolizist. Even now Germans call part of their police Kripo for Kriminalpolizei. Syllabic abbreviations were also typical of German language used in the German Democratic Republic, for example, Stasi for Staatssicherheit ("state security", the secret police and secret service) or Vopo for Volkspolizist ("people's policeman").
Some syllabic abbreviations from Russian that are familiar to English speakers include samizdat and kolkhoz. The English names for the Soviet "Comintern" (Communist International) and "Milrevcom" (Military Revolution Committee) are further examples.
Orwell's novel 1984 uses fictional syllabic abbreviations like "Ingsoc" (English Socialism) to evoke the use of language under the Nazi and Soviet regimes.
East Asian languages whose writing uses Chinese-originated ideograms instead of an alphabet form abbreviations similarly by using key characters from a term or phrase. For example, in Japanese the term for the United Nations, kokusai rengō (国際連合) is often abbreviated to kokuren (国連). Such abbreviations are called ryakugo (略語) in Japanese. SAs are frequently used for names of universities: for instance, Beida (北大, Běidà) for Peking University (Beijing), Yondae (연대) for the Yonsei University, Seouldae (서울대) for the Seoul National University and Tōdai (東大) for the University of Tokyo.

Use in various languages
Syllabic abbreviations are preferred by the US Navy as it increases readability amidst the large number of initialisms that would otherwise have to fit into the same acronyms. Hence DESRON 6 is used (in the full capital form) to mean "Destroyer Squadron 6," and DEFCON means "Defense Condition".

Syllabic abbreviations in names of organizations
In modern English there are several conventions for abbreviations and the choice may be confusing. The only rule universally accepted is that one should be consistent, and to make this easier, publishers express their preferences in a style guide. Questions which arise include those in the following subsections.

Lower case letters
A period (full stop) is sometimes written after an abbreviated word, but there is much disagreement and many exceptions.
There is never a stop/period between letters of the same word. For example, Tiberius is abbreviated as Tb. and not as T.b..
In formal British English it is more common to write abbreviations with full stops if the word has been cut at the point of abbreviation but not otherwise: for example, Street — "St[reet]" — becomes "St.", but "Saint" — "S[ain]t" — becomes "St".
In American English, the period is usually added if the abbreviation may be interpreted as a word, but some American writers do not use a period here. Sometimes, periods are used for certain acronyms but not others; a notable instance in American English is to write United States, European Union, and United Nations as U.S., EU, and UN respectively.
A third standard removes the full stops from all abbreviations (both "Saint" and "Street" become "St") .
Acronyms that were originally capitalized (with or without periods) but have since entered the vocabulary as generic words are no longer abbreviated with capital letters nor with any periods. Examples are sonar, radar, lidar, laser, and scuba.
Spaces are generally not used between single letter abbreviations of words in the same phrase, so one almost never encounters "U. S.".

Periods (full stops) and spaces
To form the plural of an abbreviation, a number, or a capital letter used as a noun, simply add a lowercase s to the end.
To form the plural of an abbreviation with periods, a lowercase letter used as a noun, and abbreviations or capital letters that would be ambiguous or confusing if the 's' alone were added, use an apostrophe and an s.
While some authors use the apostrophe in all plural abbreviated forms, it is generally best avoided except as above to prevent ambiguity with the possessive form.[1] [2] [3]

A group of MPs
The roaring '20s
Mind your Ps and Qs
A group of Ph.D.'s
The x's of the equation
Sending SOS's Plural Forms

Conventions followed by publications and newspapers
Publications based in the U.S. tend to follow the style guides of the Chicago Manual of Style and the Associated Press. The U.S. Government follows a style guide published by the U.S. Government Printing Office.
However, there is some inconsistency in abbreviation styles, as they are not rigorously defined by style guides. Some two-word abbreviations, like "United Nations", are abbreviated with uppercase letters and periods, and others, like "personal computer" (PC) and "compact disc" (CD), are not; rather, they are typically abbreviated without periods and in uppercase letters. A third variation is to use lowercase letters with periods; this is used by Time Magazine in abbreviating "public relations" (p.r.). Moreover, even three-word abbreviations (most U.S. publications use uppercase abbreviations without periods) are sometimes not consistently abbreviated, even within the same article.
The New York Times is unique in having a consistent style by always abbreviating with periods: P.C., I.B.M., P.R. This is in contrast with the trend of British publications to completely make do without periods for convenience.

In the United States
Many British publications follow some of these guidelines in abbreviation:

For the sake of convenience, many British publications, including the BBC and The Guardian, have completely done away with the use of full stops or periods in all abbreviations. These include:

• Social titles, like Ms or Mr (though these would usually not have had full stops — see above) Capt, Prof, etc.;
  Two-letter abbreviations for countries ("US", not "U.S.");
  Abbreviations beyond three letters (full caps for all except initialisms);
  Words seldom abbreviated with lower case letters ("PR", instead of "p.r.", or "pr")
  Names ("FW de Klerk", "GB Whiteley", "Park JS"). A notable exception is the newspaper The Economist which writes "Mr F. W. de Klerk".
  Scientific units.
  Acronyms are often referred to with only the first letter of the abbreviation capitalised. For instance, the North Atlantic Treaty Organisation can be abbreviated as "Nato" or "NATO", and Severe Acute Respiratory Syndrome as "Sars" or "SARS" (compare with "laser" which has made the full transition to an English word and is rarely capitalised at all).
  Initialisms are always written in capitals; for example the "British Broadcasting Corporation" is abbreviated to "BBC", never "Bbc". An initialism is similar to acronym but is not pronounced as a word.
  When abbreviating scientific units, no space is added between the number and unit (100mph, 100m, 10cm, 10ºC). (This is contrary to the SI standard, see below.) In Britain
  
  Plurals are often formed by doubling the last letter of the abbreviation. Most of these deal with writing and publishing: MS=manuscript, MSS=manuscripts; l=line, ll=lines; p=page, pp=pages; s=section, ss=sections; op.=opus, opp.=opera. This form, derived from Latin is used in Europe in many places: dd=didots. "The following (lines or pages)" is denoted by "ff". One example that does not concern printing is hh=hands.
  A doubled letter also appears in abbreviations of some Welsh names, as in Welsh the double "l" is a separate sound: "Ll. George" for (British prime minister) Lloyd George.
  Some titles, such as "Reverend" and "Honourable", are spelt out when preceded by "the", rather than as "Rev." or "Hon." respectively. This is true for most British publications, and some in the United States.
  It is usually advised to spell out the abbreviation where it is new or unfamiliar to the reader (UNESCO in a magazine about music, because it refers to the United Nations Educational, Scientific and Cultural Organization, whose work does not concern the music). Miscellaneous and general rules
  The International System of Units (SI) defines a set of base units, from which other "derived" units may be obtained. The abbreviations, or more accurately "symbols" (using Roman letters, or Greek in the case of ohm) for these units are also clearly defined together with a set of prefixes, themselves symbolised (abbreviated) with Roman letters (except only for micro, which uses the Greek letter µ), denoting powers of ten. The system is internationally recognised. Periods are not used, except as described below. Unit symbols do not have plural forms.
  Units are written either in full, including the base units and their prefixes, or with all symbols. When a unit is written in full, it is written in all lower case. For example, megaampere for MA.
  There should never be a period after or inside a unit; both '10 k.m.' and '10 k.m' are wrong — the only correct form is '10 km' (only followed with a period when at the end of a sentence).
  A period "within" a compound unit denotes multiplication of the base units on each side of it. Ideally, this period should be raised to the centre of the line, but often it is not. For instance, '5 ms' means 5 millisecond(s), whereas '5 m.s' means 5 metre·second(s). The "m.s" here is a compound unit formed from the product of two fundamental SI units — metre and second.
  There should always be a (non-breaking) space between the number and the unit — '25 km' is correct, and '25km' is incorrect.
  The case of letters (uppercase or lowercase) has meaning in the SI system, and should never be changed in a misguided attempt to follow an abbreviation style. For example, "10 S" denotes 10 siemens (a unit of conductance), while "10 s" denotes 10 seconds. Any unit named after a person is denoted by a symbol with an upper case first letter (S, Pa, A, V, N, Wb, W), but spelt out in full in lower case, (siemens, pascal, ampere, volt, newton, weber and watt). By contrast g, l, m, s, cd, ha represent gramme, litre, metre, second, candela and hectare respectively. The one slight exception to this rule is that the symbol for litre is allowed to be L to help avoid confusion with an upper case i or a one in some typefaces — compare l, I, and 1.
  Likewise, the abbreviations of the prefixes denoting powers of ten are case-sensitive: m (milli) represents a thousandth, but M (mega) represents a million, so by inadvertent changes of case one may introduce (in this example) an error of a factor of 1 000 000 000. When a unit is written in full, the whole unit is written in lowercase, including the prefix: millivolt for mV, nanometre for nm, gigacandela for Gcd.
  The above rules, if followed, ensure that the SI system is always unambiguous, so for instance mK denotes millikelvin, MK denotes megakelvin, K.m denotes kelvin.metre, and km denotes kilometre. Forms such as k.m and kms are ill-formed and technically meaningless in the SI system, although the meaning might be inferred from the context.
  
  History
  
  List of classical abbreviations
  List of medieval abbreviations
  List of abbreviations in use in 1911
  List of acronyms and initialisms
  The abbreviations used in the 1913 edition of Webster's dictionary