Several parts combine into one character.

決 is a phono-semantic character: 水 (氵, water) for meaning and 夬 (to break open) for sound. Originally breaching a blocked dike to open a water (氵) channel — from which grew "to burst, to resolve, to decide." Just as a channel once opened cannot be undone, 決 is deciding firmly.

The form splits: traditional and Japanese shinjitai are 決 (氵 + 夬), while Chinese simplified is 决, the left changed from 氵 (water) to 冫 (ice) — the left radical differs by mode.

Korean hanja-sound "gyeol": 決定 (gyeoljeong, decision), 決心 (gyeolsim, resolve), 決斷 (gyeoldan, decisiveness), 解決 (haegyeol, solution), 判決 (pan-gyeol, a verdict), 對決 (daegyeol, confrontation), 可決 (gagyeol, approval). With 定 (to determine) seen earlier it forms 決定 (decision).

Mandarin: jué, 2nd tone. 决定 (juédìng, to decide), 决心 (juéxīn, determination), 解决 (jiějué, to solve), 坚决 (jiānjué, resolute), 决赛 (juésài, the final). 解决 (jiějué, to solve) is a daily Mandarin essential.

Japanese: on-reading ケツ (ketsu), kun き.める (kimeru) and き.まる (kimaru). 決定 (kettei, decision), 決心 (kesshin, resolve), 解決 (kaiketsu, solution), 決勝 (kesshō, the final), 決める (kimeru, to decide). 決める (kimeru, to decide) is a daily Japanese essential.

Memory aid: opening a water (氵) channel (夬) — deciding firmly, traditional 決 / simplified 决.