  A general description of entanglement is suggested as an action realized by
an arbitrary operator over given disentangled states. The related entanglement
measure is defined. Because of its generality, this definition can be employed
for any physical systems, pure or mixed, equilibrium or nonequilibrium, and
characterized by any type of operators, whether these are statistical
operators, field operators, spin operators, or anything else. Entanglement of
any number of parts from their total ensemble forming a multiparticle composite
system can be determined. Interplay between entanglement and ordering,
occurring under phase transitions, is analysed by invoking the concept of
operator order indices.

Quantum Physics;Statistical Mechanics