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.
I visited the Izu Islands in the summer.
This archipelago has many volcanic islands.
The village is located in the Hawaiian archipelago.
I visited the Galápagos Islands during summer vacation.
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.