impulsive

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

• adjective Inclined to act on impulse rather than thought.
• adjective Motivated by or resulting from impulse.
• adjective Having force or power to impel or incite; forceful.
• adjective Physics Acting within brief time intervals. Used especially of a force.

from The Century Dictionary.

• Having the power of driving or impelling; moving; impellent.
• Actuated or controlled by impulses; swayed by the emotions: as, an impulsive child.
• Resulting from impulse: as, impulsive movements or gestures.
• In mech., acting by instantaneous impulse, not continuously: said of forces. See force, 8 .
• noun That which impels; impelling cause or reason.

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

• adjective Having the power of driving or impelling; giving an impulse; moving; impellent.
• adjective Actuated by impulse or by transient feelings.
• adjective (Mech.) Acting momentarily, or by impulse; not continuous; -- said of forces.
• noun That which impels or gives an impulse; an impelling agent.

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

• adjective Having the power of driving or impelling; giving an impulse; moving; impellent.
• adjective Actuated by impulse or by transient feelings.
• adjective mechanics Acting momentarily, or by impulse; not continuous – said of forces.
• adjective Highly reactive; people who react quickly or feel emotional
• noun That which impels or gives an impulse; an impelling agent.
• noun One whose behaviour or personality is characterized by being impulsive.

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

• adjective having the power of driving or impelling
• adjective characterized by undue haste and lack of thought or deliberation
• adjective proceeding from natural feeling or impulse without external stimulus
• adjective determined by chance or impulse or whim rather than by necessity or reason
• adjective without forethought