impetus

noun

1. Anything that impels; a stimulating factor.

   “The outbreak of World War II in 1939 gave a new impetus to receiver development.”

   “Once set a strong mind thinking, and you have done all that it needs for its education. It matters little what is the first impetus, so that it only be set to work.”

   “In a single moment Montenegro and their supporters were given fresh impetus and encouragement. Beciraj tested Hart with a low shot before teenager Phil Jones, on his England debut, suffered an anxious moment when Stevan Jovetic went down under his challenge, leaving the youngster clearly relieved to see referee Stark wave away Montenegro's appeals.”

2. A force, either internal or external, that impels; an impulse.
3. The force or energy associated with a moving body; a stimulus.
4. A principle of motive force, held as equivalent to weight times velocity by John Buridan, in an auxiliary theory of Aristotelian dynamics introduced by John Philoponus, describing projectile motion against gravity as linear until it transitions to a vertical drop and the intellectual precursor to the concepts of inertia, momentum and acceleration in classical mechanics.
5. An activity in response to a stimulus.