Wikipedia
Coenergy
In physics and engineering, Coenergy (or co-energy) is a non-physical quantity, measured in energy units, used in theoretical analysis of energy in physical systems.
The concept of co-energy can be applied to many conservative systems (inertial mechanical, electromagnetic, etc.), which can be described by a linear relationship between the input and stored energy.
The co-energy analysis techniques cannot be applied to non-linear systems. However, small nonlinearities are often neglected by linearisation of the problems.