A significant fraction of Swahili vocabulary derives from Arabic through contact with Persian-speaking Muslim inhabitants. For example, the Swahili word for "book" is kitabu, traceable back to the Arabic word كتاب kitābu (from the root k.t.b. "write"). However, the Swahili plural form of this word ("books") is vitabu, following Bantu grammar in which ki- is reanalysed as a nominal class prefix, whose plural is vi-.
Swahili is a Bantu language of the Sabaki branch. In Guthrie's geographic classification, Swahili is in Bantu zone G, whereas the other Sabaki languages are in zone E70, commonly under the name Nyika. Local folk-theories of the language have often considered Swahili to be a mixed language because of its many loan words from Arabic, and the fact that Swahili people have historically been Muslims. However, historical linguists do not consider the Arabic influence on Swahili to be significant enough to classify it as a mixed language, since Arabic influence is limited to lexical items, most of which have only been borrowed after 1500, while the grammatical and syntactic structure of the language is typically Bantu.