anti

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

• noun A person who is opposed to something, such as a group, policy, proposal, or practice.
• adjective Opposed.
• preposition Opposed to; against.

from The Century Dictionary.

• noun A prefix of Greek origin: originally only in compounds or derivatives taken from the Greek or formed of Greek elements, as in antipathy, antinomy, etc. (the earliest example in English being antichrist, which see), but now a familiar English formative, meaning primarily against, opposed to.
• noun One who is opposed to some proposed or undertaken course of action, policy, measure, movement, or enactment, as, for example, to imperialism.
• noun In chem., a prefix used to indicate that two groups or two atoms which might react with each other are so separated in space that they do not readily do this. It is contrasted with the prefix syn-. Thus in antibenzaldoxime, , the H and OH do not readily combine to form water, while in synbenzaldoxime, , such a combination takes place easily.

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

• A prefix meaning against, opposite or opposed to, contrary, or in place of; -- used in composition in many English words. It is often shortened to ant-.

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

• adjective chemistry Describing a torsion angle between 90° and 180°
• noun A person opposed to a concept or principle.
• preposition rare A word used before a noun or noun phrase to indicate opposition to the concept expressed by the noun or noun phrase.

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

• noun a person who is opposed (to an action or policy or practice etc.)
• adjective not in favor of (an action or proposal etc.)