abbreviation, in writing, arbitrary shortening of a word, usually by cutting off letters from the end, as in U.S. and Gen. (General). Contraction serves the same purpose but is understood strictly to be the shortening of a word by cutting out letters in the middle, the omission sometimes being indicated by an apostrophe, as in the word don't. Most abbreviations are followed by a period. Usage, however, differs widely, and recently omission of periods has become common, as in NATO and UN. Acronyms are combinations of the first letters/syllables in a group of words to form a new grouping of letters that can be pronounced as a word. A period is never used when apostrophes appear. A list of abbreviationsused in this encyclopedia may be found in the prefatory matter.