What is the Swahili word for friend?

Rafiki = friend. The plural is rafiki, same as the singular, but the new plural marafiki is growing in use.

Note: Ndugu means brother or sister or relative in general. It is often used in address to friends, ndugu yangu, my brother (relative). Under the former Tanzanian system of Ujamaa (familyhood or socialism) it became the common form of address for both sexes, like "comrade" in Communist countries. It is still so used by many people, especially older ones. The President, for example, is called Ndugu Jakaya Kikwete.

Ndugu is also the plural form, e.g., Ndugu zangu (my relatives).