isiNdebele is related to the Zulu language spoken in South Africa. This is because the Ndebele people of Zimbabwe descend from followers of the Zulu leader Mzilikazi, who left KwaZulu in the early nineteenth century during the Mfecane.
The Northern and Southern Ndebele languages are variants of the same language; they both fall in the Nguni group of Bantu languages, Northern Ndebele is essentially a dialect of Zulu, and the older Southern Ndebele language appears to be the first dialect of the Nguni language. The shared name is due to contact between Mzilikazi's people and the original Ndebele, through whose territory they crossed during the Mfecane.
The Southern Ndebele language (isiNdebele or Nrebele in Southern Ndebele) is an African language belonging to the Nguni group of Bantu languages, and spoken by the amaNdebele (the Ndebele people of South Africa).
There is also another, separate dialect called Northern Ndebele or Matabele spoken in Zimbabwe and Botswana – see Sindebele language. The Zimbabwean and South African Ndebele dialect is closer to Zulu than other Nguni dialects.