abbreviations

Definition 
Unlike acronyms, abbreviations aren't usually pronounced as words, but they do serve the same purpose as time- and space-savers. They have been popular since the earliest times, a good example being SPQR, the abbreviation for Senatus Populusque Romanus, the famous insignia of Rome. Most abbreviations merely suggest the whole word they represent to the reader (as Dr.), but many have become almost words themselves: the letters spoken, as in I.Q. for intelligence quotient. A few are even spoken as words, such as vet for veterinarian or armed service veteran, ad for advertisement, and ad lib. There are entire dictionaries devoted to the tens of thousands of abbreviations we use, and a complete list of abbreviations of government agencies can be found in the United States Government Organization Manual. Below are a handful of interesting and humorous abbreviations from slang and standard English that illustrate the diverse and complex ways such coinages are formed. Included are eusystolisms, "initials used in the interest of delicacy," such as S.O.B:

A.A. Alcoholics Anonymous
ad lib from the Latin ad libitum, at one's pleasure; was first a musical term.
C-Note century note, $100.
C.O.D. collect on delivery; has been traced back to 1859.
DTs delirium tremens.
et al. from the Latin et alia, "and others."
F.Y.I. For Your Information; ubiquitous on office memos.
G.P. general practitioner.
Ibid. from the Latin ibidem, "in the same place."
IHS the abbreviation is simply the first two letters and last letter of the Greek word for Jesus, capitalized and Romanized. It does not stand for in hoc signo ("in this sign") or any other phrase.
I.O.U. for "I owe you"; an unusual abbreviation that is based on sound, not sight.
MIG standing for a Russian jet fighter, from the initials of the designers of a series of Russian fighters.
Mrs., Mr. Mrs. originally stood for "mistress," when "mistress" meant a married woman, but since a mistress today is something entirely different, Mrs. cannot be considered a true abbreviation anymore—there is no full form for the word, unlike for Mr. (mister).
P.D.Q. stands for "pretty damn quick," e.g., "You'd better get started P.D.Q." Its origin hasn't been established beyond doubt, although it has been attributed to Dan Maguinnis, a Boston comedian appearing about 1867–1889.
Q.T. an abbreviation for "quiet"; "on the q.t." means stealthily, secretly, e.g., "to meet someone on the q.t." Origin unknown.
Q.V. from the Latin quod vide, "which see."
R.S.V.P. stands for the French répondez s'il vous plait, "please reply," "the favor of a reply is requested."
UFO Unidentified flying object, the term coined in recent times, although the first sightings of such objects were reported as far back as 1896.