The most common Korean word for “decision” is 결심 (gyeolsim). There is one more word with a similar meaning — compare them below. 「decision」는 한국어로 결심.
decision; resolution; determination
TOPIK 2 · noun
decision; crystal
TOPIK 2 · noun
Words and levels: TOPIK I essentials (National Institute of Korean Language standard curriculum, KOGL Type 1) · examples: Cheesepotato