Something that causes and encourages a given response
It is well established that Urban preached the sermon at Clermont which gave the impetus to the crusades.
This discovery gave a new impetus to prospecting in south-western Nevada, and it was soon discovered that the district was not an isolated mining region but was in the heart of a great mineral belt.
No greater impetus than this could have been given to Van Buren's candidacy for the vice-presidency.
Lannes was checked, and with his repulse the impetus of the attack died out all along the line.