Thermal equilibrium is the state a physical system is in when it has no net flow of heat to other physical systems despite them being connected by a path permeable to heat. In other words, a system is in thermal equilibrium when it isn't isolated but also transfers no heat. The process of reaching thermal equilibrium is called thermalization.
If a body is in thermal equilibrium, its thermodynamic variables are not independent of each other. They are related by the body's equation of state.