This is the 976th most frequent Chinese word.
决心
Determination or resolve.
The phrase '下定决心' means 'to make a firm decision', indicating a decisive resolution.
他终于下定决心辞掉这份工作。
He finally made the decision to quit this job.
Here '决心' is used as a noun meaning 'determination' referring to a strong mental resolve.
她有克服一切困难的决心。
She has the determination to overcome all difficulties.
In this sentence, '决心' again acts as a noun, illustrating the quality of being determined.