1a (1) : a driving force :
2: the property possessed by a moving body in virtue of its mass and its motion —used of bodies moving suddenly or violently to indicate the origin and intensity of the motion
I like to use this word as it appears in the first definition, as a sort of "cause and effect" description. For example: The man's impetus to quit smoking was the birth of his grandchild.
This is a very straightforward word, and I am not sure why it is not used more often. Perhaps you will use this as an impetus to start using it in your everyday vocabulary!