Nairobi was a city in Africa on Earth. It was the capital of the nation-state of Kenya, and later a part of the United States of Africa.
As a child, Nyota Uhura attended Public School 29 in Nairobi. (TOS novel: Vulcan's Forge) In an alternate reality, Uhura attended the Nairobi Girls Academy, where her curriculum included advanced studies of the Earth-Romulus War. (TOS - Starfleet Academy novel: The Gemini Agent)
Nairobi was the location of the University of Nairobi, where Doctor April Wade worked when she was nominated for the Carrington Award in 2371. (DS9 episode: "Prophet Motive")