combatant

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

• noun One, such as a person or a combat vehicle, that takes part in armed strife.
• adjective Engaging in armed strife.

from The Century Dictionary.

• Contending; disposed to combat or contend.
• In heraldry, same as affronté, but applied only to ferocious creatures, such as lions.
• noun A person who combats; one who engages in battle; one who fights, whether in single combat or in an army or a fleet.
• noun A person who contends with another in argument or controversy.
• noun A name of the ruff, Machetes pugnax. See ruff.
• noun In heraldry, a figure drawn like a sword-player standing upon his guard.

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

• noun One who engages in combat. IN military use, opposed to noncombatant.
• adjective Contending; disposed to contend.

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

• noun An armed fighter.

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

• adjective engaging in or ready for combat
• noun someone who fights (or is fighting)