Definition

If a single thermodynamic system (sealed and does not allow material exchange with its surroundings, and is sufficiently small relative to its surroundings), is left undisturbed for a long enough time, it will reach an equilibrium state. In this state, no macroscopic changes or tendencies toward change can be observed.

The equilibrium state of a thermodynamic system in a given environment is determined solely by the values of a set of extensive variables. In other words, if the values of these extensive variables are specified, the macroscopic properties of the system in equilibrium state are uniquely determined.

Reference

Tasaki Chapter 2 P.28