effort

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

• noun The use of physical or mental energy to do something; exertion.
• noun A difficult exertion of strength or will.
• noun A usually earnest attempt.
• noun Something done or produced through exertion; an achievement.
• noun Force applied against inertia.
• noun The force needed by a machine to accomplish work on a load.

from The Century Dictionary.

• noun Voluntary exertion; a putting forth of the will, consciously directed toward the performance of any action, external or internal, and usually prepared by a psychological act of “gathering the strength” or coördination of the powers.
• noun The result of exertion; something done by voluntary exertion; specifically, a literary, oratorical, or artistic work.
• noun In mech., a force upon a body due to a definite cause.
• To strengthen; reinforce.

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

• noun An exertion of strength or power, whether physical or mental, in performing an act or aiming at an object; more or less strenuous endeavor; struggle directed to the accomplishment of an object.
• noun (Mech.) A force acting on a body in the direction of its motion.
• transitive verb obsolete To stimulate.

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

• noun The work involved in performing an activity; exertion.
• noun An endeavour.
• verb uncommon To make an effort.
• verb obsolete To stimulate.

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

• noun earnest and conscientious activity intended to do or accomplish something
• noun use of physical or mental energy; hard work
• noun a notable achievement
• noun a series of actions advancing a principle or tending toward a particular end