The spreading of frequencies of a spectral line as a consequence of interactions between molecules. Although the term “pressure broadening” is frequently encountered and is respectable, it is misleading in that pressure per se has nothing fundamentally to do with pressure broadening. At a fixed temperature, the number density of gas molecules is proportional to pressure. The greater the number density, the smaller the average separation between molecules, and hence the greater the potential energy of intermolecular interaction. Thus pressure is merely a surrogate for interaction (collision).