When a female character isn't wearing a brassier (or any undies at all) and you can tell (either via a see-through outfit, strategically placed lace, or by use of Absolute Cleavage, Underboobs, or something backless, she's wearing mere air for underwear. In other words, she's wearing Vapor Wear.
This is the default dress code for vamps and femmes fatale. When she's wearing something significantly less vampy, the lack of underwear is usually specifically mentioned and can make even an ordinary outfit interesting.
This trope can also apply to male characters, but rarely does because the shape of a woman's nipples under her shirt is more socially acceptable than a penile bulge for some reason.