Oh gosh, it's not a dress! It's more of a Chinese robe, if you don't take the literal translation. (at least, that's what most people say as well).

As said, it's called Changshan, which literally means long shirt.

Cheongsam is not the correct term to use, technically, because cheongsam refers to the female garment. Cheongsam is actually an English name for the clothes =x Although pronouncing it in Cantonese is the same, the spelling is somewhat different.