Several parts combine into one character.

定 combines 宀 (roof) with 正 (correct, its lower part modified) — setting the place within a house (宀) rightly (正) and making it stable. From a household properly settled came "to determine, to settle, to stabilize." The 正 (correct) seen earlier sits below.

Korean hanja-sound "jeong": 決定 (gyeoljeong, decision), 安定 (anjeong, stability), 定義 (jeongui, definition), 豫定 (yejeong, schedule), 確定 (hwakjeong, confirmation), 規定 (gyujeong, regulation). It enters nearly every word about "fixing" something.

Mandarin: dìng, 4th tone. 决定 (juédìng, to decide), 一定 (yídìng, definitely), 定义 (dìngyì, definition), 稳定 (wěndìng, stable), 规定 (guīdìng, regulation), 肯定 (kěndìng, certainly). 一定 (yídìng, definitely) is a daily Mandarin essential.

Japanese: on-readings テイ (tei) and ジョウ (jō), kun さだ.める (sadameru) and さだ.まる (sadamaru). 決定 (kettei, decision), 予定 (yotei, schedule), 安定 (antei, stability), 定食 (teishoku, a set meal), 定規 (jōgi, a ruler — a ジョウ reading). The 定食 (teishoku) at a Japanese diner carries this character.

Memory aid: setting the inside of a house (宀) right (正) — settled, hence determined and stable.