Jaati
Jaati means a classification based on source of origin. Nyaya Sutra states “Samaanaprasavaatmika Jaatih” or those having similar birth source form a Jaati.
An initial broad classification made by Rishis is 4-fold: Udbhija (coming out of ground like plants), Andaja (coming out of eggs like birds and reptiles), Pindaja (mammals) and Ushmaj (reproducing due to temperature and ambient conditions like virus, bacteria etc).
Similarly, various animals like elephant, lion, rabbits etc form different ‘Jaati’. In same manner, entire humanity forms one ‘Jaati’. A particular Jaati will have similar physical characteristics, cannot change from one Jaati to another and cannot cross-breed. Thus Jaati is creation of Ishwar or God.
Brahmin, Kshatriya, Vaishya and Shudra are no way different Jaati because there is no difference in source of birth or even physical characteristics to differentiate between them.
Later, word ‘Jaati’ started being used to imply any kind of classification. Thus in common usage, we call even different communities as different ‘Jaati’. However that is merely convenience of usage. In reality, all humans form one single Jaati.