The P'o (pronounced roughly p-ho), is the lower soul of a creature. The exact nature of a P'o varies based on the game:
Wraith: The Oblivion
The P'o is a specific attribute of Asian ghosts in Wraith: The Oblivion, it is effectively an Asian wraith's Shadow, but is generally more tradition bound and more concerned with protection of the body rather than the active nihilism of its western counterpart.
Kindred of the East
In Kindred of the East, a Kuei-Jin's P'o is an archetypal personality, serving the same function as a shadow. In addition, all characters have a P'o attribute, which governs the strength of the lower soul relative to the higher soul.
Exalted
In Exalted, the P'o is primarily a plot device, in that it serves to motivate hungry ghosts and to protect the tomb but does not actively take a role in the game.