大海|たいかいtaikai[0]heiban (flat)

1. nounocean; large sea
   そのボートは大海たいかいを漂ただよっていた。↗

   The boat was drifting in the ocean.
2. nounfabric pattern

• Synonym海うみNeutralThe everyday word for the sea and the most general choice; 「大海」 adds a sense of vast, boundless expanse, as in the idiom 「井の中の蛙大海を知らず」.
• Synonym海洋かいようWrittenWritten and formal, often scientific; mostly seen in compounds like 「海洋学」. Use it in formal prose, not casual talk about the sea.
• Synonym大洋たいようWrittenA geographic term for the oceans — the Pacific, the Atlantic and so on; more formal than 「大海」.
• Synonym海原うなばらWrittenA literary, poetic word for a boundless stretch of sea; 「大海」 is the more neutral way to say it.
• SynonymオーシャンNeutralA loanword for the ocean, mostly in names and branding such as 「オーシャンビュー」 rather than everyday speech.
• Synonym大海原おおうなばらWrittenA literary word for the vast ocean, laying even more stress on breadth than 「海原」; close to 「大海」 in sense.
• Related外洋がいようWrittenThe open sea far from land, contrasted with 「内海」; it stresses position rather than the headword's sense of vastness.
• Related遠洋えんようWrittenFar-off seas, stressing distance from land; common in set phrases like 「遠洋漁業」 (distant-water fishing).
• Related遠海えんかいWrittenA written word for seas distant from land, but rarely used; say 「遠洋」 instead.
• Related外海がいかいNeutralThe open sea outside bays and inlets, as in 「外海に出る」; it marks position relative to the coast, not vastness.

For the sea in general, say 「海」. Use 「海洋」 in formal or scientific writing, often in compounds. To stress vastness pick 「大海」 or 「大海原」, as in 「大海に漕ぎ出す」; for waters far from land, 「遠洋」. The others are rare or specialized.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 広大こうだいな海原うなばら。大洋たいよう。