noun

• (collective, geography) A group of islands.
  "For many years past the whaleship has been the pioneer in ferreting out the remotest and least known parts of the earth. She has explored seas and archipelagoes which had no chart, where no Cook or Vancouver had ever sailed."
• (now rare, specifically) The Aegean Sea.
  "[I]n his imagination he had settled his route, through Holland and France to Sicily, which he had long wished to see, and from thence to the Archipelago […]."
• (by extension) Something scattered around like an archipelago.
  "The Gulag Archipelago"

verb

• (transitive, rare) To scatter or be scattered so as to resemble a group of islands.
  "As the seas level, as the seas / Swept into ripples by the breeze, / And archipelagoed by trees, / Majestic spreading oaks, that rise / Like island walls against the skies."