archipelago; group of islands
A geographical noun for multiple islands considered as one group. It is often attached to a place name, as in 伊豆諸島 or フィリピン諸島.
夏に伊豆諸島を訪れた。
I visited the Izu Islands in the summer.
The village is located in the Hawaiian archipelago.
この諸島には火山島が多い。
This archipelago has many volcanic islands.
I visited the Galápagos Islands during summer vacation.
列島 describes islands strung out in a long chain (as in 日本列島), whereas 諸島 refers to a cluster of islands grouped together without implying a linear shape.
島々 is a plain, everyday way to say 'various islands,' while 諸島 is the formal term attached to a specific geographic name.
A Sino-Japanese compound of 諸, meaning 'various' or 'many,' and 島, meaning 'island'; together it refers to multiple islands as a group.