Mom·ba·sa (mŏm-bās'ə, -bä'sä) A city of southeast Kenya mainly on Mombasa Island, in the Indian Ocean north of Zanzibar. Visited by Vasco da Gama on his first voyage to India (1498), the island-city was ruled successively by Portugal, Oman, Zanzibar, and Great Britain until the early 20th century. Population: 799,000.