An archipelago, more simply known as an island chain, chain of islands, or island group, was a collection of islands concentrated in a localized region of an ocean.
The unidentified planet that the Retellian escape pod carrying Trip Tucker and First Monarch Kaitaama flew towards in 2153 consisted primarily of water, but had a chain of islands located near the equator that Tucker piloted towards for them to land on. (ENT: "Precious Cargo")
The hydrothermal pools of Ren-gham were located by a lava tube near an adjacent pyroclastic debris field on the polar island chain, 227 kilometers from the southern tip of the Rykos shoreline of an unidentified planet. (ENT: "Bounty")
The bulk of radiation that leaked from an unnamed garbage scow that threatened Gamelan V in 2367 was projected to affect three small sparsely populated island groups on the planet. (TNG: "Final Mission")