noun, modal particle · a firm purpose or resolution to achieve a goal

他下定了决心要改掉这个坏习惯。

adverb · to make up one's mind or decide firmly on a course of action

我们决心要把这项科研任务攻克下来。

adjective, noun · possessing or displaying a firm and resolute character or attitude

既然已经下定了学习的决心，就不要轻易放弃。