A geographical term for a sea area containing many islands, or the island group itself. More formal than 群島 (ぐんとう).
The Aegean Sea is known as an archipelago.
Compound of 多 (た, 'many'), 島 (とう, 'island'), and 海 (かい, 'sea'). The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning 'archipelago'.