For the archetype commonly abbreviated BA, see Burning Abyss.
Ka and Ba
Ba, or Spirit Energy, is a person's life force. When someone Summons or commands a Ka, they lose Ba. If someone loses all their Ba, they die (get sent to the Shadow Realm in the dub). The strength of one's Ka depends on the strength of their Ba.
In the anime, the amount of Ba is shown on the Eye of Wdjat of the user's DiaDhank.
LP are the modern day equivalent of Ba.