A thermodynamic property is a macroscopic characteristic of a system, such as mass, volume, pressure, and temperature, to which a numerical value can be assigned at a given time without needing to know the system’s previous behavior or history.

Thermodynamic properties or variables are often categorized as being either extensive and intensive.

Definition of extensive variable

A thermodynamic property is extensive if its value for the entire system is the sum of the values for the parts into which the system is divided. In other words, extensive variable is proportional to the size or the extent of the system.

Common examples of extensive properties include:

• Volume $V$
• Amount of substance $N$
• Internal energy $U$
• Helmholtz energy $F$
• Gibbs energy $G$
• etc.

For convenience a set of extensive variables is denoted by $X$, e.g. $X=(V,N)$.