A system may be defined as "any physical object of interest". It may be an open system, having a fixed boundary and variable mass, or a closed system, having fixed mass and variable volume.
The boundary of a system defines the system, and may or may not be part of it. Outside the boundary lie the surroundings.
A property of a system is any physical variable that can be measured. It may be either intensive or extensive. Intensive properties are independent of the size of the system; examples are density and temperature. Extensive properties vary according to the size of the system; examples are mass and volume. A property that is independent of the path between two fixed points is called a function of state.