noun
main island
Refers to the principal island of a group, such as Okinawa's main island or Japan's Honshu in context.
沖縄本島は那覇空港がある島です。
Okinawa's main island is the island where Naha Airport is located.
台風は本島に接近しています。
The typhoon is approaching the main island.
noun
this island
Used when the speaker is on the island they are referring to; context-dependent and less common than この島 in casual speech.
本島の人口は約十万人です。
The population of this island is about 100,000.