Definition
The pressure produced at the bottom of a column of fluid by the weight of the fluid above it. The taller the column, the greater the pressure at the base. Often expressed as a height of fluid (for example, feet of water or inches of mercury) rather than as a force per unit area.
Plain English
The push you get at the bottom of a tank or pipe just because there is fluid stacked above it. More fluid above means more push at the bottom.
Context Anchor
Seen in fuel, oil, and liquid-pressure system discussions, especially when explaining how gravity can help move fluid through a line.
Derivation
‘Head’ here comes from an old engineering use meaning the height or top level of a body of fluid. So ‘head of pressure’ literally means ‘the pressure created by the height of the fluid.’ Knowing this makes the term easier to picture: it is height being expressed as pressure.
Why Pilots Care
In gravity-fed fuel systems and similar setups, the head of pressure is what keeps fluid flowing to the engine or component without a pump. If the supply tank is too low, or the line is routed poorly, the head of pressure may not be enough to keep things working as expected.
Analogy
Think of a tall water tower. The water at the bottom of the building feels strong pressure not because anything is pumping it, but simply because there is a tall column of water stacked above it. That is head of pressure.
Intuition Check
“Head” does not mean the front or top part of something here. Here it means the height of liquid that creates pressure by pressing downward.
Example Sentence 1
The fuel tank is mounted high in the wing so that the head of pressure feeds the carburetor by gravity alone.
Example Sentence 2
Technicians calculate the head of pressure when checking oil flow through the engine's lubrication system.