  Interaction with environment may lead to the transition of quantum system
from pure state to the mixed one. In this case, the problem of definition of
entanglement may arise. In particular, quantitative measure of entanglement
concurrence is introduced differently for pure and mixed states. To solve this
problem, alternative definition of concurrence of mixed states is introduced in
present work. In particular, concurrence of mixed states is determined as an
average of pure states concurrences. Averaging is carried out over the quantum
ensemble, corresponding to all possible realization of random variable.
