Indonesian (bahasa Indonesia, [baˈha.sa in.doˈne.sja]) is the official language of Indonesia. It is a standardised variety of Malay, an Austronesian language that has been used as a lingua franca in the multilingual Indonesian archipelago for centuries. Indonesia is the fourth most populous nation in the world—of which the majority speak Indonesian, which makes it one of the most widely spoken languages in the world.

Most Indonesians, aside from speaking the national language, are fluent in at least one of the more than 700 indigenous local languages; examples include Javanese, Sundanese, and Balinese, which are commonly used at home and within the local community. However, most formal education and nearly all national mass media, governance, administration, and judiciary and other forms of communication are conducted in Indonesian.

The term “Indonesian” is primarily associated with the national standard dialect (bahasa baku). However, in a more loose sense, it also encompasses the various local varieties spoken throughout the Indonesian archipelago. Standard Indonesian is confined mostly to formal situations, existing in a diglossic relationship with vernacular Malay varieties, which are commonly used for daily communication.

[Source]