Batak is a collective term used to identify a number of ethnic groups predominantly found in North Sumatra

Sumatra

Sumatra is an island in western Indonesia, westernmost of the Sunda Islands. It is the largest island entirely in Indonesia , and the sixth largest island in the world at 473,481 km2 with a population of 50,365,538...

, Indonesia

Indonesia

Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...

. The term is used to include the Toba

Batak Toba people

The Batak Toba people, also called Toba people or often simply 'Batak', are the most numerous of the Batak people of North Sumatra, Indonesia, and often considered the classical 'Batak', most likely to willingly self-identify as Batak. The Batak Toba people speak in the Toba Batak language and are...

, Karo, Pakpak, Simalungun

Simalungun people

The Simalungun people are an ethnic group in North Sumatra, considered one of the Batak peoples. Simalungun people live mostly in Simalungun Regency and the surrounding areas, including the city of Pematang Siantar, an autonomous city, but previously part of Simalungun Regency.The Simalungun live...

, Angkola and Mandailing

Mandailing

The Mandailing is a traditional cultural group in Southeast Asia. They are found mainly in the northern section of the island of Sumatra in Indonesia. They came under the influence of the Kaum Padri who ruled the Minangkabau of Tanah Datar. As a result, the Mandailing were influenced by Muslim...

, each of which are distinct but related groups with distinct, albeit related, languages and customs (adat

Adat

Adat in Indonesian-Malay culture is the set of cultural norms, values, customs and practices found among specific ethnic groups in Indonesia, the southern Philippines and Malaysia...

). Occasionally it is also used to include the Alas people of Central/Southern Aceh

Aceh

Aceh is a special region of Indonesia, located on the northern tip of the island of Sumatra. Its full name is Daerah Istimewa Aceh , Nanggroë Aceh Darussalam and Aceh . Past spellings of its name include Acheh, Atjeh and Achin...

, but usually only as relates to language groups.

In North Sumatra, Toba people typically assert their identity as 'Batak', while other 'Bataks' may explicitly reject that label, preferring instead to identify as specifically 'Simalungun', 'Karo', etc.
Linguistic and archeological evidence indicates that Austronesian speakers first reached Sumatra from Taiwan

Taiwan

Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...

 and the Philippines

Philippines

The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...

 through Borneo and/or Java about 2,500 years ago, and the Batak probably evolved from these settlers.