一定
いってい · ittei · JLPT N2 · noun
regular
📌 一定 = one + settle: kept at one fixed level. Ittei sounds like it stay
一 means one and 定 means settle or fix, as in 定食, the set meal. Something 一定 is settled at a single level, as in 一定の速度, a steady speed. Note: in Chinese, 一定 usually means certainly, but in Japanese it means fixed or regular.
一 — single horizontal stroke
一 is one of the simplest pictographs in Chinese, depicting a single horizontal line to represent the number one. It has retained essentially the same form since oracle bone script over 3,000 years ago. By extension it also means 'first,' 'whole,' or 'same.'
定 — roof 宀 + 正 (correct/upright, phonetic)
Originally written with 宀 (roof) over 正 (upright/correct), suggesting a household settled and orderly under its roof. Over time 正 simplified to 㐬-like form at the bottom, and the meaning shifted from 'settled home' to the broader senses of 'fix, stable, decide.'