ism

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A distinctive doctrine, system, or theory.

from The Century Dictionary.

• noun A doctrine, theory, system, or practice having a distinctive character or relation: chiefly used in disparagement: as, this is the age of isms; to set up an ism.
• noun A suffix implying the practice, system, doctrine, theory, principle, or abstract idea of that which is signified or implied by the word to which it is subjoined: as, dogmatism, spiritualism, socialism, Atticism, Americanism, Gallicism, terrorism, vandalism, republicanism, Mormonism, being especially common in nouns so formed from names of persons and designating theories, as Benthamism, Comtism, Darwinism, etc., or theories associated with practice, especially in words of temporary use, as Cæsarism, Jacksonism, Grantism, etc., such temporary words being formed as occasion requires, in unlimited numbers. Such words are usually accompanied by a noun of the agent in -ist, and an adj. in -istic, and often by a verb in -ize. See these suffixes.

from the GNU version of the Collaborative International Dictionary of English.

• noun A doctrine or theory; especially, a wild or visionary theory.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A belief that can be described by a word ending in -ism.
• noun Specifically, a form of discrimination, such as racism or sexism.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a belief (or system of beliefs) accepted as authoritative by some group or school