The column of fluid in pipes is elastic and affected by both weight and friction. These characteristics are described in terms of the pipe capacity, the pipe inductance and the flow resistance (effective resistance).
The column of fluid represents a homogeneous oscillating system as opposed to a discrete mechanical one. In a system of this kind, variable pressure and flow rate are dependent on both time and place. The corresponding form of oscillation is referred to as a wave. Inside pipes, given the small diameter compared with the relevant wavelengths, it can be assumed there will be some one-dimensional propagation of oscillation. Pipe impedance (also known as characteristic impedance) refers to the wave resistance inside a pipe.