Equilibrium Internal and External

When we refer to a property of a system - say pressure or temperature -we assume that the system is uniform and, therefore, this property reflects the whole system. 

A system within which there are no non-uniformities, that act as driving forces for change, is said to be in a state of internal equilibrium. This is the type of systems that we will be dealing with in this book (see Example 1.2.c). 

if there are no driving forces that can bring about a change between a system and its surroundings, the system is said to be in a state of external equilibrium. 

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