The term HEMI references the hemispherical
combustion chambers in the engine's heads.
The chamber design puts the intake/exhaust
valves in-line, rather than side-by-side. This
allows for better flow of air through the head.
The spark plug in the center of the chamber
creates a better ignition of the fuel/air. Both
of these major aspects help make hemi-type
engines produce more efficiency and power.
While Mopar is not the only company to have
used a hemi-type engine, they have been the
most successful at mass-producing them for
the general public (and for the racing scene).