Excluded volume is a basic type of interaction between particles. In essence, two particle cannot overlap, which leads to a repulsive force between particles. The Van der Waals gas model, for example, can be derived using excluded volume and statistical mechanics. Aside from just the physical presence of a molecule leading to excluded volume, it is also often of merit to consider effective exclude volume. For example, we could use the setup below to setup an effective excluded volume for a rod molecule, which can rotate with varying degrees of freedom depending on the density of particles.
Richard A.L. Jones, Soft Condensed Matter', pg. 64, Oxford University Press (2002).
Figure from AP 284 Statistical Thermodynamics, Prof. Amir Yacoby, HW 5 Prob 3 (2009).