Yes, Hemi refers to the domed shape of the engine’s cylinder head. Half spherical. The thing about hemis is that the nature of the shape of the chamber forces you to splay the valves out so they fall on opposite sides of the cylinder, producing a very large cylinder head. The shape of other cylinder heads are known as “wedge heads” because both valves are aligned in the same direction producing a wedge shape.
Chrysler developed it back in the early fifties, and they produced quite a bit more power than the extant engines of the time, due to the efficient nature of the Hemi. The tradeoff of course is that hemis are (usually) much heavier than wedge heads because of the huge cylinder head arrangement. (think of those characteristic wide valve covers with the plug wire emerging from the middle.
But, hot rodders discovered quickly where hemis REALLY shine. Supercharging. Hemis took quite well to forcing large amounts of fuel/air mixture into the cylinders without detonating. Plus the engines themselves were quite stout and stood up to the abuse quite nicely. For a while, at least.