Yoruba is not a tribe, it is a nation made up of different tribes with the same history, source, basic language similiarities, folklore, customs and tradition.
The term Yoruba was the word used to address the citizens of the Oyo empire. The first textual occurence of the word Yoruba was found in the texts of Songhai scholars from Songhai empire who came to the Oyo empire around the 15th century A.D.
The british were the ones who popularized the use of the word Yoruba to categorize every other Omoluabi tribes. The Ijebus, the Egbas, Ekitis, Yewa, Oyo, Ife, Ijesha, Ibolo, Owo, Akoko, Itsekiris, Ondo, Igbomina, Oworo, Awori, Ilaje, Ikale, Ketus, Sabes, Oku, Aku, Ga’s, okun and olukumi are all independent tribal units within the Yoruba commonwealth.
All this Yoruba tribes are spread out in the following west African countries namely, Gambia, Sierra leone, Liberia, Ghana, Ivory coast, Togo, Benin Republic and Nigeria as indigenous people.