A compartment is an amount of some material that is homogeneous. Examples might be, the amount of a drug in the blood stream of a person, or the number of infected individuals in an epidemic of a specific disease in a uniform population. A compartment is usually represented diagramatically by a square or a circle, with arrows pointing towards and away from the compartment, as shown in Figure 1.
Figure 1
The arrows represent flows of material, into or out of the compartment.