self-denial; self-restraint; spirit of self-control
A somewhat formal or literary term for the mental attitude of restraining one's own desires or impulses. Often used in contexts of moral discipline or personal development.
Without self-denial, you cannot achieve a big goal.
He overcame temptation with self-restraint.
克己 is a shorter, more abstract noun meaning self-denial or self-conquest. 克己心 adds 心 (heart/spirit) to emphasize the mental attitude.
Compound of 克己 (self-denial, from 克 'overcome' and 己 'self') and 心 (heart, spirit). The exact historical derivation is uncertain, but the word is conventionally associated with the concept of moral self-discipline.