Archipelago
Etymology
From Italian "arcipelago," from Greek "arkhi-" ("chief") and "pelagos" ("sea"). It originally named the Aegean Sea specifically; because that sea is dotted with islands, the word shifted to mean any island group.
noun · /ar-kih-PEL-uh-goh/
A group or chain of islands scattered across a stretch of sea.
What it names
An island cluster — with a surprising etymology that began as the name of a particular sea, not the islands in it.
Example
The archipelago stretched for hundreds of miles across the Pacific.
Related idea
An atoll is one particular kind of island found in an archipelago — a ring of coral enclosing a lagoon.