Thermodynamic equilibrium is when the inputs and outputs of a system is equal. This may be in terms of heat energy or in terms of liquid/gas flow.

In thermal equilibrium, and only in thermal equilibrium, entropy is constant.

S. Srinivasan has written:

'Simplified curve fits for the thermodynamic properties of equilibrium air' -- subject(s): Curve fitting, Equilibrium air, Thermodynamic properties

The quasi-balanced state of a thermodynamic system near to thermodynamic equilibrium, in some sense. In a quasistatic or equilibrium process, a sufficiently slow transition of a thermodynamic system from one equilibrium state to another occurs such that at every moment in time the state of the system is close to an equilibrium state. During a quasistatic process, the system reaches equilibrium much faster, almost instantaneously, than its physical parameters vary.

You can search the relevant meaning of the availability condition for thermodynamic system in the wikipedia since there's so much to learn and laws of thermodynamic equilibrium that explains what and those condition works.