Several parts combine into one character.

帝 originated as a pictograph of an altar with offerings — possibly flowers or sacrificial vessels suspended. The original referent in Shang dynasty oracle bones was actually the highest deity (天帝 / 上帝, "Heavenly Lord"), the supreme god worshipped by the Shang ruling house. Only later did the term migrate from the divine to the human ruler — when human kings claimed cosmic authority, they took the name of the high god as their title. Hence "emperor" inherited a religious origin in CJK political vocabulary.

Korean reading "je." 皇帝 (hwangje, emperor — used for foreign emperors and historical Chinese rulers), 帝王 (jewang, monarch / sovereign), 上帝 (sangje, "Supreme Lord" — used in early Korean Christian Bible translations as "God"), 先帝 (seonje, "previous emperor" — historical), 女帝 (yeoje, empress regnant). Korean historical fiction uses 제 vocabulary widely.

Mandarin dì, 4th tone. 帝 (dì), 皇帝 (huángdì, emperor — paired compound; 秦始皇帝 Qín Shǐhuángdì "First Emperor of Qin" was the first to take this combined title, in 221 BCE), 上帝 (Shàngdì, "Supreme Lord" — adopted by Chinese Bible translators as the standard term for the Christian God; the alternative is 神 shén), 天帝 (tiāndì, Heavenly Emperor / supreme deity in Chinese folk religion). The Mandarin 帝 retains both political and religious resonance.

Japanese on-reading テイ (tei) — 帝王 (teiō, monarch), 皇帝 (kōtei, emperor — used for foreign emperors like Napoleon, Roman emperors, and Chinese emperors), 帝国 (teikoku, empire — 大日本帝国 Dai-Nippon Teikoku, "Empire of Greater Japan," was Japan's name 1868-1947), 女帝 (jotei, empress regnant — applied historically to Empress Suiko and others). Importantly, Japan does NOT use 帝 to refer to the Japanese imperial sovereign — that title is 天皇 (Tennō, "Heavenly Sovereign"). The 帝 / 皇 distinction in Japanese is significant: 帝 is for foreign or historical emperors, 皇 is for the unique Japanese imperial line.

Memory aid: an altar with offerings — origin as the supreme heavenly deity later became the title of human emperors who claimed cosmic authority.