Heart radical 忄 + 亘 (continuous) = 'a heart that holds steady' — the core meaning is constancy. High-frequency compounds: 永恒 yǒnghéng 'eternal', 恒星 héngxīng 'fixed star (star)', 恒心 héngxīn 'perseverance', 持之以恒 chízhī yǐ héng 'to persevere'.
history
恒 was the original name of Mt. Heng (恒山) in Shanxi, the Northern Sacred Peak (北岳) of the five sacred mountains, and gives its name to Hong Kong's Hang Seng Index (恒生指数, 'lasting prosperity').
忄 is the left-side form of 心 and marks 恒 as a mental/character word — "perseverance" lives in the heart. Same radical in 情, 怀, 怕, 慢 — all interior-state vocabulary.
亘 supplies the sound, drifting from gèn to héng. It also adds semantic flavour — 亘 means "spanning continuously" (亘古 since antiquity), echoing the "constant, unbroken" sense of 恒. Phonetic + faint semantic resonance.
