Maid costume and apron dress
Originally, maid costume is a common term used to refer to the maid clothes or the dresses that was made in imitation of the maid clothes, worn by servants and housekeepers in Britain at the end of the 19th century. Today, this type of dress is commonly called as "Classical maid costume", and is not mainstream maid outfits for itself.
But as for the apron dress, worn in Britain at the time, are generally defined as the symbolic outfit of Japanese maids. Originally, the apron dress was not limited as the maid costume but worn generally. Taking a look at famous example, Alice, the heroine in "Alice's Adventures in Wonderland(1865)", is also clothed in the apron dress.
The reason why the apron dresses become to be defined as the symbolic outfit of Japanese maids has not be made clear yet. It may be that the pure white apron dresses provide Japanese Otaku guys with both comfort and Moe exciting feeling as if the virtual lover in video games really come into being in front of them.