Medical Definition of TSETSE FLY. : any of several dipteran flies of the genus Glossina that occur in sub-Saharan Africa and include vectors of human and animal trypanosomes (as those causing sleeping sickness and nagana)—called also tsetse.ull Definition of TSETSE FLY. : any of several dipteran flies (genus Glossina) that occur in Africa south of the Sahara and include vectors of human and animal trypanosomes —called also tsetse — compare sleeping sickness. See tsetse fly defined for English-language learners. See tsetse fly defined for kids.