stimulant

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

• noun An agent, especially a chemical agent such as caffeine, that temporarily arouses or accelerates physiological or organic activity.
• noun A stimulus or an incentive.
• noun A food or drink, especially an alcoholic drink, believed to have a stimulating effect.
• adjective Serving as or being a stimulus; stimulating.

from The Century Dictionary.

• Stimulating; serving to stimulate, incite, or provoke; specifically, in physiology, temporarily quickening some functional or trophic process.
• noun That which stimulates, provokes, or incites; a stimulus; a spur.
• noun In physiology, an agent which temporarily quickens some functional or trophic process.

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

• adjective Serving to stimulate.
• adjective (Physiol.) Produced increased vital action in the organism, or in any of its parts.
• noun That which stimulates, provokes, or excites.
• noun (Physiol. & Med.) An agent which produces a temporary increase of vital activity in the organism, or in any of its parts; -- sometimes used without qualification to signify an alcoholic beverage used as a stimulant.

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

• noun A substance that acts to increase physiological or nervous activity in the body.
• noun Something that promotes activity, interest, or enthusiasm.
• adjective Acting as a stimulant.

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

• adjective that stimulates
• noun a drug that temporarily quickens some vital process
• noun any stimulating information or event; acts to arouse action