noun, noun or participle which takes 'suru'
momentous decision; crucial decision; big resolution
A yojijukugo used when someone makes a very important, life-changing decision or resolution. Often implies a dramatic, once-in-a-lifetime determination.
彼は一大決心をして会社を辞めた。
He made a momentous decision and quit his job.
留学するために一大決心した。
I made a big resolution to study abroad.