A wholehearted resolution to do something; a determined decision to change or begin anew—often used for a turning point or decisive resolve.
彼は一念発起して毎日ジョギングを始めた。
He resolved to change and started jogging every day.
大学受験のために一念発起し、勉強を本格化させた。
He made a firm decision to take the university exam and intensified his studies.
一念発起して禁煙を決めたが、最初の一週間が一番大変だった。
He resolved to quit smoking, but the first week was the hardest.