The Nepalese caste system is complex and continues the traditional system of social stratification of Nepal. The caste system defines social classes by a number of hierarchical endogamous groups often termed as Jāt. This custom was previously only prevalent in the core Hindu societies of the Khas, Newar, Madhesi. However, since the unification of Nepal in the 18th century, Nepal's various indigenous tribes have been incorporated within the caste hierarchy level. Nepalese caste system broadly borrows the classical Hindu chaturvarnashram model consisting of four broad social classes or varna: Brahmin, Kshatriya, Vaishya and Sudra. The ethnic indigenous groups do not belong to this class system.