A named division of a Māori iwi (tribe), membership is determined by genealogical descent; a hapū is made up of a number of whānau (extended family) groups. Te Maire Tau noted in his study of Ngāi Tahu migrations that hapū size and names were volatile with hapū splitting into sister groups when they grew in size or when migrating. New hapū were often named after events associated with the migration. Likewise the same group of people would change their name according to different circumstances. These were primarily to do with rights to resources that had been given to a named hapū or to link to an ancestor with mana in a particular area. Tau states that hapū names and locations have become more stable in more recent times. Generally hapū range in size from 350–500 although there is no upper limit. A Māori person can belong or have links to many different hapū. Each hapū had its own chief and normally operated independently of the tribe (iwi) group. Missionaries such Henry Williams noted that even in times of war against another iwi, hapū usually operated independently. In the Musket Wars many of the battles were fights between competing hapū rather than iwi. It was not uncommon for two hapū from the same iwi to be in conflict.
Hapū frequently were the political unit that sold land to the Europeans. In the 20 years after the Treaty of Waitangi Native Affairs Minister Chris Richmond said that about half of all sales under the Treaty of Waitangi, were by different hapū or comparatively small groups of individuals. He said that all the land sold north of Auckland, some in Hawkes Bay, in the Wairarapa valley, Waikato at Raglan, and by Te Āti Awa in Wellington and Taranaki was by hapū or small group.
Before the arrival of Europeans the normal day-to-day operating group seems to have been the smaller whānau (extended family). By the 1820s Māori had learnt the benefit of working in larger groups especially when it came to trading with ships. The larger hapū could work more effectively to produce surplus flax, potatoes, smoked heads and pigs in exchange for blankets, tobacco, axes and trade muskets. In warfare the hapū was the standard grouping for warriors during the musket war period. Hapū would unite politically under their own chief, to form much larger armies up to several thousand warriors, although it was common for hapū to retain independence within the larger group.
The literal meaning of the word is "pregnant" which is a metaphor for the genealogical connection that unites the members of the hapū. Similarly, the Māori word for land, whenua, can also mean "placenta", metaphorically indicating the connection between the people and the land.
See also 
- "Tribal organisation", Te Ara
- rt-1-traditional-maori-concepts "Traditional Maori Concepts", Ministry of Justice
- "How iwi and hapū were named", Te Ara
- "Tribal organisation", Te Ara
- Ngāi Tahu, A Migration History. Editors Te Maire Tau and Atholl Anderson. Bridget Williams. Wellington 2008, pp. 20–23
- Appendix to Journals.1861,E-01,page 26,supplementary to Governor's Despatch.
- "...hapū means both pregnant and clan...", Te Ara