The English noun "will" has a few different meanings. One is the choice or intention to do something, as in "willing" and "free will": Latin voluntās.
Another meaning, though, is more like "resolve" or "mental strength": the force of mind that drives one to do something despite difficulties (as in "willpower", "force of will", "will to live").
How would I express this second meaning in Latin? For example, someone might have the desire to start something, but lack the willpower to follow through on it. What is the Latin word for what they lack?