In an internal combustion engine, the cylinder head (often informally abbreviated to just head) sits above the cylinders on top of the cylinder block. It closes at the top of the cylinder, forming the combustion chamber.
A head gasket seals this joint. In most engines, the head also provides space for the passages that feed air and fuel to the cylinder and allow the exhaust to escape. The chair can also be a place to mount valves, spark plugs, and fuel injectors.