An impi is a Zulu word for any armed body of men. It originates in historic tribal warfare, where groups of armed men called impis battled one another. The term if commonly associated with the rise of the Zulu nation under Shaka.
Shaka was a gifted warrior and war strategist. He introduced a number of innovations to the traditional Zulu fighting style, including use of a short stabbing spear, rather than the traditional throwing assegai, the use of a larger, stronger shield, and barefoot fighting to improve speed. As King, he introduced a far more systematic and brutal approach to warfare, that quickly saw massive consolidation of Zulu power. By the time of Shaka's assassination in 1828, the Zulu kingdom was the greatest power in southern Africa.
Any grouping of men on a mission could collectively be called an impi. The size of the impi might range from 40 to 40,000, depending on the purpose of the mission and the number of soldiers available. The Zulu impi at which defeated the British at Isandhlwana numbered 40,000 men.