What is "head gasket"

n. a gasket to seal a cylinder head

A head gasket is a gasket that sits between the engine block and cylinder head(s) in an internal combustion engine.

Its purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders; as such, it is the most critical sealing application in any engine, and, as part of the combustion chamber, it shares the same strength requirements as other combustion chamber components.