Several parts combine into one character.

皇 is a compound ideograph: 白 (originally a depiction of a flame, later "white") + 王 (king / sovereign). The composite reads as "the king who shines forth like a flame" = great ruler, emperor, sovereign. 皇 forms the canonical pair with 帝, and the compound 皇帝 was first adopted in 221 BCE by 秦始皇帝 (Qin Shi Huangdi), the First Emperor of unified China. He combined two existing terms — 三皇 (the Three Sovereigns) and 五帝 (the Five Emperors) of legendary antiquity — into a new title claiming greater authority than any previous king.

Korean reading "hwang." 皇帝 (hwangje, emperor — formal compound), 皇帝陛下 (hwangje pyeha, "His Imperial Majesty"), 皇族 (hwangjok, imperial family / imperial relatives), 皇室 (hwangsil, imperial household), 女皇 (yeohwang, empress regnant). Korean uses 황 for foreign emperors and ancient Chinese rulers; the Joseon dynasty rulers were 王 (wang, king), with the brief Korean Empire (대한제국, 1897-1910) being the only period Korean rulers took the 皇帝 title.

Mandarin huáng, 2nd tone. 皇 (huáng), 皇帝 (huángdì, emperor), 皇宫 (huánggōng, imperial palace — Beijing's Forbidden City is referred to as 故宫 Gùgōng "former palace" today, but historically the 皇宫), 皇后 (huánghòu, "after the emperor" = empress / wife of the emperor), 教皇 (jiàohuáng, "religious-emperor" = pope — Catholic vocabulary). 皇 sits at the apex of the imperial hierarchy.

Japanese on-reading コウ (kō) is the dominant reading and carries unique cultural weight: 天皇 (Tennō, "Heavenly Sovereign" = Emperor of Japan — the title of the Japanese imperial sovereign, an unbroken hereditary line traditionally counted from Emperor Jimmu in 660 BCE). The English "Emperor of Japan" translates Tennō, not 皇帝. 皇室 (kōshitsu, imperial family), 皇族 (kōzoku, imperial relatives), 皇位 (kōi, imperial throne), 皇居 (Kōkyo, the Imperial Palace in Tokyo — the residence of the current Tennō). Alternative on-reading オウ (ō) appears in 法皇 (hōō, "dharma sovereign" = retired emperor who took Buddhist vows — historical Japanese position). For Japanese, 皇 is one of the most reverent characters in the entire writing system.

Memory aid: 帝 + 皇 = 皇帝, the imperial title coined by China's First Emperor in 221 BCE. Japan uses 皇 for its own sovereign (天皇), 帝 for foreign or historical emperors.