Abbreviation

An abbreviation is a shortened form of a word or phrase, by any method including shortening, contraction, initialism, or crasis. An abbreviation may be a shortened form of a word, usually ended with a trailing full stop. For example, the term etc. is the usual abbreviation for the Latin phrase et cetera.

Types

A contraction is an abbreviation formed by replacing letters with an apostrophe. Examples include I'm for I am and li'l for little.
An initialism or acronym is an abbreviation consisting of the initial letter of a sequence of words without other punctuation. For example, FBI, USA, IBM, BBC. When initialism is used as the preferred term, acronym refers more specifically to when the abbreviation is pronounced as a word rather than as separate letters; examples include SWAT and NASA.
Initialisms, contractions and crasis share some semantic and phonetic functions, and are connected by the term abbreviation in loose parlance.

History

In early times, abbreviations may have been common due to the effort involved in writing or to provide secrecy via obfuscation.
Reduction of a word to a single letter was common in both Greek and Roman writing. In Roman inscriptions, "Words were commonly abbreviated by using the initial letter or letters of words, and most inscriptions have at least one abbreviation". However, "some could have more than one meaning, depending on their context. " Many frequent abbreviations consisted of more than one letter: for example COS for consul and COSS for its nominative etc. plural consules.
Abbreviations were frequently used in early English. Manuscripts of copies of the Old English poem Beowulf used many abbreviations, for example the Tironian et or for and, and for since, so that "not much space is wasted". The standardisation of English in the 15th through 17th centuries included a growth in the use of such abbreviations. At first, abbreviations were sometimes represented with various suspension signs, not only periods. For example, sequences like were replaced with, as in for master and for exacerbate. While this may seem trivial, it was symptomatic of an attempt by people manually reproducing academic texts to reduce the copy time.
In the Early Modern English period, between the 15th and 17th centuries, the thorn was used for th, as in . In modern times, was often used for promotional reasons, as in.
During the growth of philological linguistic theory in academic Britain, abbreviating became very fashionable. Likewise, a century earlier in Boston, a fad of abbreviation started that swept the United States, with the globally popular term OK generally credited as a remnant of its influence.
Over the years, however, the lack of convention in some style guides has made it difficult to determine which two-word abbreviations should be abbreviated with periods and which should not. This question is considered below.
Widespread use of electronic communication through mobile phones and the Internet during the 1990s led to a marked rise in colloquial abbreviation. This was due largely to increasing popularity of textual communication services such as instant and text messaging. The original SMS supported message lengths of 160 characters at most, for instance. This brevity gave rise to an informal abbreviation scheme sometimes called Textese, with which 10% or more of the words in a typical SMS message are abbreviated. More recently Twitter, a popular social networking service, began driving abbreviation use with 140 character message limits.
In HTML, abbreviations can be annotated using abbreviation to reveal its meaning by hovering the cursor.

Style conventions in English

In modern English, there are multiple conventions for abbreviation, and there is controversy as to which should be used. One generally accepted rule is to be consistent in a body of work. To this end, publishers may express their preferences in a style guide.
Some controversies that arise are described below.

Capitalization

If the original word was capitalized then the first letter of its abbreviation should retain the capital, for example Lev. for Leviticus. When a word is abbreviated to more than a single letter and was originally spelled with lower case letters then there is no need for capitalization. However, when abbreviating a phrase where only the first letter of each word is taken, then all letters should be capitalized, as in YTD for year-to-date, PCB for printed circuit board and FYI for for your information. However, see the following section regarding abbreviations that have become common vocabulary: these are no longer written with capital letters.

Full Stops (Periods)

A period is sometimes used to signify abbreviation, but opinion is divided as to when and if this convention is best practice.
According to Hart's Rules, a word shortened by dropping letters from the end terminates with a full stop, whereas a word shorted by dropping letters from the middle does not. Fowler's Modern English Usage says a full stop is used for both of these shortened forms, but recommends against this practice. It advises it only for end-shortened words and lower-case initialisms, not for middle-shortened words and upper-case initialisms.

Full form	Shortening	Short form	Source
Doctor	mid	Dr	D——r
Professor	end	Prof.	Prof...
The Reverend	end	Rev.	Rev...
The Reverend	mid	Revd	Rev——d
The Right Honourable	mid and end	Rt Hon.	R——t Hon...

Some British style guides, such as for The Guardian and The Economist, disallow periods for all abbreviations.
In American English, the period is usually included regardless of whether or not it is a contraction, e.g. Dr. or Mrs. In some cases, periods are optional, as in either US or U.S. for United States, EU or E.U. for European Union, and UN or U.N. for United Nations. There are some house styles, however—American ones included—that remove the periods from almost all abbreviations. For example:

The U.S. Manual on Uniform Traffic Control Devices advises that periods should not be used with abbreviations on road signs, except for cardinal directions as part of a destination name.
AMA style, used in many medical journals, uses no periods in abbreviations or acronyms, with almost no exceptions. Thus and hundreds of others contain no periods. The only exceptions are , to avoid confusion with the word "No"; initials within persons' names ; and "St." within persons' names when the person prefers it .

Acronyms that were originally capitalized but have since entered the vocabulary as generic words are no longer written with capital letters nor with any periods. Examples are sonar, radar, lidar, laser, snafu, and scuba.
When an abbreviation appears at the end of a sentence, only one period is used: The capital of the United States is Washington, D.C.
In the past, some initialisms were styled with a period after each letter and a space between each pair. For example, U. S., but today this is typically US.

Plural

There are multiple ways to pluralize an abbreviation. Sometimes this accomplished by adding an apostrophe and an s, as in "two PC's have broken screens". But, some find this confusing since the notation can indicate possessive case. And, this style is deprecated by many style guides. For instance, Kate Turabian, writing about style in academic writings, allows for an apostrophe to form plural acronyms "only when an abbreviation contains internal periods or both capital and lowercase letters". For example, "DVDs" and "URLs" and "Ph.D.'s", while the Modern Language Association explicitly says, "do not use an apostrophe to form the plural of an abbreviation". Also, the American Psychological Association specifically says, "without an apostrophe".
However, the 1999 style guide for The New York Times states that the addition of an apostrophe is necessary when pluralizing all abbreviations, preferring "PC's, TV's and VCR's".
Forming a plural of an initialization without an apostrophe can also be used for a number, or a letter. Examples:

For units of measure, the same form is used for both singular and plural. Examples:

1 lb or 20 lb
1 ft or 16 ft
1 min or 45 min

When an abbreviation contains more than one period, Hart's Rules recommends putting the s after the final one. Examples:

Ph.D.s
M.Phil.s
The d.t.s

However, the same plurals may be rendered less formally as:

PhDs
MPhils
The DTs

According to Hart's Rules, an apostrophe may be used in rare cases where clarity calls for it, for example when letters or symbols are referred to as objects.

The x's of the equation
Dot the i's and cross the t's

However, the apostrophe can be dispensed with if the items are set in italics or quotes:

The xs of the equation
Dot the 'i's and cross the 't's

In Latin, and continuing to the derivative forms in European languages as well as English, single-letter abbreviations had the plural being a doubling of the letter for note-taking. Most of these deal with writing and publishing. A few longer abbreviations use this as well.

Singular abbreviation	Word/phrase	Plural abbreviation	Discipline
d.	didot	dd.	typography
f.	following line or page	ff.	notes
F.	folio	Ff.	literature
h.	hand	hh.	horse height
J.	Justice	JJ.	law
l.	line	ll.	notes
MS	manuscript	MSS	notes
op.	opus	opp.	notes
p.	page	pp.	notes
Q.	quarto	Qq.	literature
s.	section	ss.	notes
v.	volume	vv.	notes