Define penetration.

Hi

I was recently challenged to define Penetration.
We failed to find a suitable definition - although I have my own opinions.

Views presented on the subject include:
Glider penetration is represented by the polar curve.
Penetration is proportional to wing loading.
Penetration is the "real world L/D" taking into account the energy
reserves over drag to overcome upsets caused by turbulence etc.
Penetration is related to weight only. (variant ratio of- weight and
frontal area * Coefficient of Drag)

I know we must be missing the point so -
Any takers from our learned aerodynamicist types?

What exactly is "penetration" as applied to gliders and how does it
differ from the polar curve.