From Old Japanese *umi*, attested in the earliest texts including the Man'yōshū (8th century). Related to the verb *umu* (to give birth), reflecting an ancient conception of the sea as a great nurturing source. The kanji 海 combines 水 (water) with a phonetic component 每, which also appears in 母 (haha, mother) — reinforcing the "life-giving waters" imagery.

Japan is an island nation (archipelago of ~6,800 islands), so 海 is deeply woven into everyday life, folklore, and identity. Summer (夏, natsu) is synonymous with beach trips — going to 海水浴 (kaisuiyoku, sea bathing) is a quintessential seasonal activity. The word appears in the famous phrase 山より海派 (yama yori umi-ha, "I'm a sea person rather than a mountain person"), a common icebreaker. Note: 海 (umi) means the sea as a natural body; 洋 (yō) appears in compound words for ocean regions (太平洋, Taiheiyō = Pacific Ocean).