The Nilotic meaning people from the Nile Valley. Nilo Saharans are however found all over Eastern Region, from Eritrea (Kunama people), Upper nile (Nubia people) & Eastern parts of Libya. Nilotic is an umbrella for a language group of people whose origin stem from Nile-valley.
While Nubia, Kunama, Luo, Kanuri, anuak, fur, ngambay among others are tribes with distinct cultures and languages within that group. While the ancient egyptians & kush kingdom refered to the God of the niles (god of floods on the nile) as HAPI, most nilotic/nilo-saharans till this date refer to water as Api/pii/hapii depending on what region you live in.