Hemi is a nickname for hemispherical.

Combustion chambers in heads have some shape. All kinds of shapes have been tried over the years. Since a sphere is the smallest surface area for a given volume, it was natural for someone to try a domed or "hemispherical" chamber layout. It offered advantages in theory since the spark plug could be located in the top of the "dome" and the flame travel would be somewhat equal in all directions. Putting the valves in a single row created layout problems in a hemi (or domed) chamber so two rows were used, one for intakes and one for exhausts. The hemispherical chamber was supposedly more efficient in the days before swirl designs, etc The original hemi motors from the early fifites were impressive with their wide valve covers (to hide two rocker shafts).

Wedge is another combustion chamber shape. It allows the valves to all be in a single row....simpler and cheaper to build. Chevrolet and Ford engines have wedge chambers.

Chrysler made both styles of heads and used them on engines that had 426 cubic inches. The heads are not interchangeable.

Now, the word HEMI is a marketing ploy....just like Super Sport, Cobra Jet, etc.....the new hemi shares Zero parts with a real hemi Chrysler....in the late sixties, saying "it's a hemi!" meant something......instead of just a big plastic badge on the fender.