Just to add to the definition...JayHäss said:Hemi is short for Hemisphere, as in half a sphere.
Back in the 60's where high technology was having more than one carburator, Chrysler made a head with the chamber shaped like half a sphere and spark plug in the middle. It's better for combustion and makes more power. It was the big technological "oooooooo" of the time.
Basically all DOHC cars now are hemi's but with 4 valves.