A "sheer top" refers to a piece of clothing, usually a blouse or a shirt, that is made from a translucent or transparent fabric. It allows light to pass through, making it partially see-through. Full definition
Oh, and I should clarify that statement a bit: they are having fun wearing sheer tops with nothing but a bra underneath. (fashables.com)
I never really thought of sheer top layers as a trend, but for as long as I can remember, sheer fabrics have been a part of my style. (creativefashionglee.com)
I'm in love with the floral embroidery design on your black sheer top! (hautekhuutureblog.com)