Mental or physical energy and willpower; vitality or the strength of one's resolve.
病気で気力がなく、起き上がるのもつらかった。
I had no energy due to illness and even getting up was hard.
彼は最後まで気力を振り絞って走り切った。
He mustered all his willpower and finished the run.
試験勉強のために気力を維持するのは難しい。
It's difficult to keep up the energy and motivation for exam study.