A traditional nun's habit, specifically a traditional Christian style (Catholic, Orthodox, etc), as opposed to the fetishized, non-traditional nun habits typically seen in anime and fanart.
It consists at most of a long dress (that's black) with long sleeves and a guimpe (that's white), and typically stars the distinctive nun headdress (consisting of a wimple/coif [that's white], headwear bandeau [that's white], and veil [that's black], though it has become increasingly optional for some since the 20th century), and may also include a tabard, underdress, and so on. The habit is typically white-on-black, with black the predominant color, but they can also be wholly white, or white-on-blue. Color can depend on the religious order which the nun is a part of. They can also be wholly black, which is the common choice for Orthodox nuns.
The traditional habit can be accentuated with minor additions, but not so much that it detracts from the basics (ex. Sister Claire and Double as opposed to Specter and Index), and some degree of modesty must be adhered to (ex. not too tight-fitting, no shortened dresses, no cutouts, no openings where they wouldn't normally be, no side slits which reveal skin or undergarment, no overtly sexual depictions of certain undergarments, etc).
