Ningen is a polite way to describe physical beings that are not Ovaforms. Generally speaking, there is usually serious avoidance of using the word "Mortal" because it implies unequal status. Aeternalae society as a whole -- that is, most factions -- agreed on Ningen as the best possible inoffensive term.
It derives from the word Ningenkai, which is from Japanese myth that translates roughly to "The world where humans live".
Generally if a being is self-aware ("sentient") it counts as Ningen in the eyes of the Aeternalae.