Wikipedia
Nusantara is a contemporary Indonesian term for the Indonesian archipelago. It originated in Old Javanese and literally means " archipelago". In Malay, Nusantara means the Malay world.
The word Nusantara was taken from an oath by Gajah Mada in 1336, as written in the Old Javanese Pararaton and Nagarakretagama. Gajah Mada was a powerful military leader and prime minister of Majapahit who was credited with bringing the empire to its peak of glory. Gajah Mada delivered an oath called Sumpah Palapa, in which he vowed not to eat any food containing spices until he had conquered all of Nusantara under the glory of Majapahit.
Today, Indonesian historians believed that the concept of Nusantara was not an idea coined by Gajah Mada for the first time in 1336. It was coined earlier in 1275 as Cakravala Mandala Dvipantara by Kertanegara of Singhasari. Dvipantara is a Sanskrit word for the "islands inbetween", the synonym to Nusantara as both dvipa and nusa mean "island". The term is used to describe the Southeast Asian Archipelago. Kertanegara envisioned the union of Southeast Asian maritime kingdoms under Singhasari as a bulwark against the rise of the expansionist Mongol Yuan dynasty in mainland China.