What Is a (Pressure) Wavefront?

In the context of two- and three-dimensional wave propagation, the word ‘wavefront’ is used to describe a surface that is expanding in time, and in the corresponding two-dimensional case, it describes a line that is also expanding in time. The equivalent definition in one-dimensional applications is simply a point, but the word ‘wavefront’ is nevertheless commonly used in the context of fluid waves in pipes, albeit for a very different purpose. However, no formal definition exists in the 1D context and the word is not interpreted consistently by all authors, thereby creating a potential for miscommunication that is clearly undesirable in scientific or engineering contexts. This paper uses a wide range of one-dimensional, wave-like phenomena to illustrate how varied wave propagation can be, even when the initial conditions are undisturbed and the trigger of the disturbance is simple. Despite these simplifications, no universally meaningful, formal definition of a wavefront seems to be achievable, partly because, in some instances, key flow parameters such as pressure and velocity exhibit completely different behaviour. Accordingly, instead of seeking a universal, unambiguous definition, a qualitative definition is proposed that is consistent with most of the presented examples and with the most common uses of the word in the general literature.