An abbreviation is a short way of writing a word or a phrase that could a
be written out in full. So, for example, you might write Dr Kinsey inste
Doctor Kinsey. Here Dr is an abbreviation for the word Doctor. Likewise, the
phrase for example can sometimes be abbreviated to e.g.
Abbreviations must be clearly distinguished from contractions. The key difference is that an abbreviation does not normally have a distinctive pronunciation of its own. So, for example, the abbreviation Dr is pronounced just like Doctor, the abbreviation oz is pronounced just like ounce(s) and the abbreviation e.g. is pronounced just like for example. (True, there are a few people who actually say "ee-jee" for the last one, but this practice is decidedly unusual.) A contraction, in contrast, does have its own distinctive pronunciation: for example, the contraction can't is pronounced differently from cannot, and the contraction she's is pronounced differently from she is or she has.
Abbreviations are very rarely used in formal writing. Almost the only ones which are frequently used are the abbreviations for certain common titles, when these are used with someone's name: Mr Willis, Dr Livingstone, Mrs Thatcher, Ms Harmon, St Joan. (Note that the two items Mrs and Ms are conventionally treated as abbreviations, even though they can be written in no other way.) When writing about a French or Spanish person, you may use the abbreviations for the French and Spanish equivalents of the English titles: M. Mitterrand, Sr. González. (These are the usual French and Spanish abbreviations for Monsieur and Señor, equivalent to English Mister.) Observe that each of these abbreviations begins with a capital letter.
Other titles are sometimes abbreviated in the same way: Prof. Chomsky, Sgt. Yorke, Mgr. Lindemann. However, it is usually much better to write these titles out in full when you are using them in a sentence: Professor Chomsky, Sergeant Yorke, Monsignor Lindemann. The abbreviated forms are best confined to places like footnotes and captions of pictures.
Note carefully the use of full stops in these abbreviations. British usage favours omitting the full stop in abbreviations which include the first and last letters of a single word, such as Mr, Mrs, Ms, Dr and St; American usage prefers (A) Mr., Mrs., Ms., Dr. and St., with full stops. Most other abbreviated titles, however, require a full stop, as shown above.
A person's initials are a kind of abbreviation, and these are usually followed by full stops: John D. Rockefeller, C. Aubrey Smith, O. J. Simpson. Increasingly, however, there is a tendency to write such initials without full stops: John D Rockefeller, C Aubrey Smith, O J Simpson. And note the rare special case illustrated by Harry S Truman: the S in this name never takes a full stop, because it's not an abbreviation for anything; President Truman's parents actually gave him the middle name S.
Two other common abbreviations are a.m. (`before noon') and p.m. (`after noon'): 10.00 a.m., six p.m. These are always acceptable. Note that these are not capitalized in British usage (though American usage prefers (A) 10.00 am and six pm, with small capitals and no full stops).
Also usual are the abbreviations b.c. and a.d., usually written in small capitals, for marking dates as before or after the birth of Christ:
Non-Christians who do not use the Christian calendar may prefer to use b.c.e. (`before the common era') and c.e. (`of the common era') instead. This is always acceptable:
Note also that, when an abbreviation comes at the end of a sentence, only one full stop is written. You should never write two full stops in a row.
Many large and well-known organizations and companies have very long names which are commonly abbreviated to a set of initials written in capital letters, usually with no full stops. Here are a few familiar examples:
A few other abbreviations are so well known that you can use them safely in your writing. Every reader will understand what you mean by GCSE examinations (GCSE = General Certificate of Secondary Education), or by DDT (dichlorodiphenyltrichloroethane), or by IQ (intelligence quotient), or by FM radio (FM = frequency modulation). Indeed, in some of these cases, the abbreviated form of the name is far more familiar than the full name.
Otherwise, however, you should try to avoid the use of abbreviations in your formal writing. The frequent use of unnecessary abbreviations will make your text irritating and hard to read. So, you should write four ounces (not 4 oz.), 80 miles per hour (not 80 mph), the Church of England (not the C of E), the seventeenth century (not C17 or the 17th cent.) and the second volume (not the 2nd vol.) It is far more important to make your writing easy to read than to save a few seconds in writing it.
There is one exception to this policy. In scientific writing, the names of units are always abbreviated and always written without full stops or a plural s. If you are doing scientific writing, then, you should conform by writing 5 kg (not 5 kilogrammes, and certainly not *5 kg. or *5 kgs.), 800 Hz (not 800 Hertz) and 17.3 cm3 (not 17.3 cubic centimetres).
There are a number of Latin abbreviations which are sometimes used in English texts. Here are the commonest ones with their English equivalents:
The abbreviation ca. `approximately' is properly used only in citing a date which is not known exactly, and then usually only if the date is given in parentheses:
Outside of parentheses, you should usually avoid the use of ca. and prefer an English word like about or approximately:
The abbreviation etc. calls for special comment. It should never be used in careful writing: it is vague and sloppy and, when applied to people, rather offensive. Do not write something like this:
Finally, there are two further (and highly objectionable) Latin abbreviations ibid. and op. cit.
Observe that it is usual to write Latin abbreviations in italics, but this is not strictly essential, and many people don't bother.
There has recently been a fashion in some circles for writing Latin abbreviations without full stops, and you may come across things like ie and eg in your reading. I consider this a ghastly practice, and I urge you strongly not to imitate it. (Note, however, that et al. has only one full stop, since et `and' is a complete word in Latin.)
One final point: very many people who should know better use the Latin abbreviation cf., which properly means `compare', merely to refer to published work. It is now very common to see something like this:
Summary of abbreviations: