Definition
The cylinder head is the upper, closed end of an engine cylinder that seals the combustion chamber and houses the intake valve, exhaust valve, and spark plugs. In an air-cooled aircraft engine, it is finned on the outside to shed heat into the passing airflow.
Plain English
It's the cap that closes off the top of each cylinder. The fuel-air mixture burns just below it, and it holds the valves and spark plugs that make that burn happen.
Context Anchor
Seen in aircraft engine descriptions, maintenance discussions, and temperature indications related to engine cooling.
Derivation
Head' is used here in its old mechanical sense of 'the closed end' of a vessel or container -- the same sense as the head of a barrel. It tells you which end of the cylinder you're looking at: the sealed top end, not the open bottom where the piston travels.
Why Pilots Care
Condition of the cylinder head directly affects compression, cooling, and power output; damage can cause engine failure.
Analogy
It is somewhat like a strong lid on the top of a metal tube, but it does more than cover the tube: it also holds important engine parts and helps carry heat away.
Intuition Check
Do not read “head” as a person’s head or as just a rounded end. In this engine context, the cylinder head is a specific metal engine part that closes the top of the cylinder and helps make power possible.
Example Sentence 1
During the climb, the pilot opened the cowl flaps to keep the cylinder head temperatures from climbing into the red.
Example Sentence 2
A mechanic removed the rocker covers to inspect the cylinder head for cracks during the annual inspection.