In Ancient Egyptian religion, the term denoting the soul. Objects were also believed to have a ka, and the ka-part of food and drink were consumed by the human ka. Thus the purpose of sacrifices of food to the spirits of the dead: they would devour the ka-element, leaving the physical food intact — which doubles as a clever explanation of why the food doesn't actually go anywhere when sacrificed.
It was formerly often translated as »double«, but this is now considered incorrect (as far as I know; maybe the fashion in Egyptology has changed again): a mistake provoked by the iconographic habit in tomb decoration of depicting the deceased's ka as a second copy of him standing next to the original. When I was a child, it was still common even in books for kids to read various convoluted explanations of this idea as a ghost-bro or proxy that accompanied the living person at a distance of about three feet and then flew away when he died, but as I say, all that is obsolete for the time being and the consensus is that it's just a plain old soul like an Abrahamic person would expect to have. A brittle consensus, however; for when there isn't enough new or unexamined material available, professional scholars have to justify their existence by inventing new theories about and arguing over the existing stuff.
Many other elements or representations of the Egyptian spirit also exist, especially in a funerary context, since Egyptian notions of the transition to the afterlife evolved to a notable complexity, but the moment of death was understood to be the moment when the ka leaves the body.