impetus

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

• noun An impelling force; an impulse.
• noun The force or energy associated with a moving body.
• noun Something that incites; a stimulus.
• noun Increased activity in response to a stimulus.

from The Century Dictionary.

• noun Energy of motion; the power with which a moving body tends to maintain its velocity and overcome resistance: as, the impetus of a cannon-ball; hence, figuratively, impulse; impulsion; stimulus.
• noun In gunnery, the altitude due to the first force of projection, or the space through which a body must fall to acquire a velocity equal to that with which a ball is discharged from a piece.
• noun The sudden force of passion.

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

• noun A property possessed by a moving body in virtue of its weight and its motion; the force with which any body is driven or impelled; momentum.
• noun Fig.: Impulse; incentive; stimulus; vigor; force.
• noun (Gun.) The altitude through which a heavy body must fall to acquire a velocity equal to that with which a ball is discharged from a piece.

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

• noun Something that impels, a stimulating factor.
• noun A force, either internal or external, that impels; an impulse.
• noun The force or energy associated with a moving body; a stimulus.
• noun An activity in response to a stimulus.

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

• noun the act of applying force suddenly
• noun a force that moves something along