foe

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

• noun A personal enemy or opponent.
• noun One who is opposed to an idea or cause.
• noun An enemy in war.
• noun Something that is destructive or injurious.

from The Century Dictionary.

• noun An enemy; one who entertains hatred, grudge, or malice against another.
• noun An enemy in war; one of a nation or people at war with another, whether personally inimical or not; a hostile or opposing army; an adversary.
• noun An opponent; a malevolent or hostile agent or principle: as, a foe to all measures of reform; intemperance is a foe to thrift.
• noun One who or that which injures, harasses, or hinders anything: as, the climate is a foe to grape-culture.
• noun Synonyms Antagonist, Opponent, etc. See adversary.

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

• noun One who entertains personal enmity, hatred, grudge, or malice, against another; an enemy.
• noun An enemy in war; a hostile army.
• noun One who opposes on principle; an opponent; an adversary; an ill-wisher; as, a foe to religion.
• transitive verb obsolete To treat as an enemy.

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

• noun A unit of energy equal to 1044 joules.
• adjective obsolete Hostile.
• noun An enemy.

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

• noun an armed adversary (especially a member of an opposing military force)
• noun a personal enemy