Prefab is a short, everyday clip of prefabricated. As a noun, a prefab is a small building made from ready-made pieces and put up quickly: a site office, a temporary classroom, a simple house. After wars, whole streets of prefabs went up as cheap homes, so the word can still feel a little “temporary / not fancy,” though some modern prefabs are well made. As an adjective, prefab housing means the same idea. Informal. The long, careful word is prefabricated.