A hemi is an engine with hemi-spherical combustion chambers – in other words, the combustion chamber is half a sphere.

This shape of combustion chamber provides space for large valves (improving volumetric efficiency) and encourages complete combustion of the fuel (improving thermal efficiency).

A true hemi-spherical combustion chamber is very large by modern standards, limiting the compression ratio which can be obtained. So modern engines often use combustion chambers which are less than half a sphere, and these are more accurately described as part-spherical.

Hemi can also refer specifically to a series of V8 engine families built by Chrysler, beginning in 1951.