Any Regency romance you read will at one point or another (be it an erotica or not) mention the all important chemise. I don't know why, it's like using bra in a contemporary. Women wear them, but unless the sentence reads She tossed her bra or she ran out of the house without her bra I can't imagine why contemporary readers would need to know that a modern day woman is wearing her bra.
Regency women knew they wore a chemise--there's no reason to actually mention such a thing. But we 21st century writers need to. And often.
So what is a chemise?
From Lord Scott's We Make History:
Chemise: The chemise was the only ladies' undergarment used during the era. (Panties would not be developed until the 20th century and pantalets were not in vogue until Victorian times.) The chemise was simply constructed of linen or cotton. In modern terms its appearance was similar to a long blouse or short nightgown.