An abbreviation (from Latin brevis, meaning “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:
There are four main kinds of abbreviations:
shortenings, contractions, initialisms, and acronyms.
1- shortenings: Shortenings of words usually consist of the first few letters of the full form and are usually spelled with a final period when they are still regarded as abbreviations.
for example: cont. = continued, in = inch.
NOTE: In the cases when they form words in their own right, the period is omitted,
for example: hippo = hippopotamus, limo = limousine.
Such shortenings are often but not always informal. Some become the standard forms, and the full forms are then regarded as formal or technical,
for example: bus = omnibus, taxi = taxicab, deli = delicatessen, zoo = zoological garden.
NOTE: Sometimes shortenings are altered to facilitate their pronunciation or spelling:
bike = bicycle
2- Contractions: Contractions are abbreviated forms in which letters from the middle of the full form have been omitted,
for example: Dr. = doctor, St. = saint or street.
NOTE: Such forms are invariably followed by a period.
Another kind of contraction is the type with an apostrophe marking the omission of letters: can’t = cannot, didn’t = did not, you’ve = you have.
3- Initialisms: Initialisms are made up of the initial letters of words and are pronounced as separate letters: CIA (or C.I.A.), pm (or p.m.), U.S. (or US).
Practice varies with regard to periods, with current usage increasingly in favor of omitting them, especially when the initialism consists entirely of capital letters.
4-Acronyms Acronyms are initialisms that have become words in their own right, or similar words formed from parts of several words. They are pronounced as words rather than as a series of letters,
for example: AIDS, laser, scuba, UNESCO, do not have periods.
NOTE: In many cases the acronym becomes the standard term and the full form is only used in explanatory contexts.
Source: Microsoft® Encarta