Words that are commonly used in The Kane Chronicles series.
The ka is the life force, the part of your soul that keeps you alive and leaves your body when you die.
The ib is the heart, the record of a person's good and bad deeds. It is weighed on scales by Anubis when a spirit is being judged for the afterlife. If it is heavier than the feather of truth, then Ammit the Devourer will eat their heart and the spirit will cease to exist. Alternatively, it has been said in The Serpent's Shadow that a spirit can be retrieved after it has been destroyed by using its shadow, though how this is done is unknown.