定 is built from the 宀 (roof) and 正 components.
It reads ジョウ like 正, as do 政, 証, 従, and 症 — learn 正 once and the family's reading comes with it.
A common word using 定 is 予定 (よてい), meaning “plans, arrangement, schedule”.
Kanji that look similar include 宗, 官, 宙, and 宜.
