frilled hairband
A hairband with frills. This may be as simple as a ribbon across the top of the head with frills on either side like in Danbooru post #3040277 or Danbooru post #3516242. The Fleur de Lapin uniform notably includes a frilled hairband, and Alice Margatroid wears one as part of her normal costume.
Hairbands with a single upright frill (Danbooru post #2661396) should be tagged maid headdress whether the character is a maid or not.
A frilled hairband with additional embellishments like embroidery or laces (Danbooru post #2366937, Danbooru post #3379468) might be a lolita hairband.