proprium

n., pl., -pri·a (-prē-ə).
In Aristotelian thought, a predicable property common to all members of a kind but not constituting part of the definition of that kind.

[Medieval Latin, from neuter of Latin proprius, proper (to) (translation of Greek idion).]

In Aristotle, any property belonging to all and only things of a certain kind, but not part of their essence. Being the only creature that wears clothes is a proprium of human beings.