Definition of "impi"

And in the end he, as well as three of his four children, shared Retief's fate in the massacre of Trekkers by the Zulu impis of Dingane.

1978, André Brink, Rumours of Rain, Vintage, published 2000, page 212

But as grizzled as Chief Joyi often seemed, the decades fell off him when he spoke of the impis, or warriors, in the army of King Ngangelizwe.

1994, Nelson Mandela, Long Walk to Freedom, Abacus, published 2010, page 26