No, an archipelago is not a music theory term; it is a geographic term for a group or chain of islands in close proximity to each other.
Some of the most well known examples include Indonesia and the Philippines, while well known countries that many might not realize are archipelago's include Japan, New Zealand and the United Kingdom. There are many archipelagos in the world, including Vanuatu, Solomon Islands, South Orkney Islands, Åland Islands, Canary Islands, Seychelles Islands, Florida Keys, Aleutian Islands, Balearic Islands and many more. Some, like the Ryukyu Islands, are a part of larger archipelagos; the Ryukyu islands being a part of Japan. Even freshwater lakes can have archipelago's, Lake Erie has the uncreative yet effectively named Lake Erie Islands. Here are the maps of two archipelagos:
Map of the Canary Islands. Map credit: Wikipedia user: Mysid
1916 map of the Western Aleutian Islands in what was then the Alaska Territory.