The concept of energy head.
The unit of energy is the same as that of work and, in the S.I. system, it is the Joule. One Joule is equal to one Newton metre that is the work done when a force of one Newton moves through one metre. Each term of the energy equation is in energy/unit weight so they all have units of Joules/Newton. However these units can also be expressed as Newton metres/Newton which reduces to metres. This should not be unexpected as the potential energy/unit weight is in metres.
There are many systems in engineering where the flow starts from a free surface and the energy available in the system depends on the elevation of the free surface above some relevant datum level. This elevation is called the head and is quite clearly equal to the potential energy/unit weight. As potential energy/unit weight can be exchanged for kinetic energy/unit weight or for pressure energy/unit weight it is just a short step to talking of kinetic head and pressure head. The concept of head is convenient because it is shorter to write. Unhappily, when calculations of power are involved, it is easy to forget that head is really Joules/Newton and when I make such calculations I revert to the basic units rather than metres of head.