A system Altered in time by the environment
The symbolism again comes from the language of quantum mechanics. The Rho (subscript s) and U have their same meaning. But now a second density operator is present, the environment (env). The entity is irreversibly altered by the interaction with the environment, generally leading to a loss of coherence and entanglement. If the two density operators are separable and not entangled, the system will remain essentially unchanged.