I am an engineer. "Head" is a a term of pressure. There are components to it though. There is static head, pressure head, and dynamic head. A head of steam refers mainly to pressure head. This is the pressure of steam in the steam drum of a pressure (usually propulsion for trains or steam ships) boiler after it has been fired to anything above atmospheric pressure, and thus has the ability to do work. There are other things called "friction head", "dynamic head" and "static head" that are also common commonly measured in terms of feet. The "feet" means the feet of height of a column of liquid of the same temperature of the fluid whose pressure is being measured. Head is used extensively in engineering terms when specifying boilers, pumps, etc, and also in the design of piping systems which need to account for "head loss" due to pipe friction from the fluid moving inthe pipes. Old steamship engineers like myself used the term occasionally for one main purpose and that was when raising steam pressure to it's design pressure for the boiler before using it to roll the turbines. And I am not that old, so maybe that explains why we never really used that term to much. We had a big old pressure gauge that read in PSIG that told us what we needed to know.