For tagging purposes this tag is for any image depicting someone wearing a western-style maid uniform, regardless of gender.
A maidservant (or in current usage, maid) is a female employed in domestic service. Once part of an elaborate hierarchy in great houses, today the maid may be the only domestic worker that upper and even middle-income households can afford. Maids perform typical domestic chores such as cooking, ironing, washing, cleaning the house, grocery shopping, walking the family dog, and taking care of children.
Anime maids typically wear some variation of the classic French maid uniform or English maid uniform (the former is primarily decorative, with frills, short skirt, and bib apron, while the latter is usually an ankle-length or mid-calf length dark-colored dress with a pinafore apron). The most indicative sign that a woman is a maid is a white head-dress and some sort of white apron dress.
If a character that is not usually portrayed with a maid uniform is drawn with one, also add the enmaided tag.