To frit is to fuse glass ingredients to prepare to turn them into glass.
But fritted glass isn’t just glass. According to Audobon:
Fritted glass has granules fused onto it to create a frost-like pattern that reduces its reflectivity and transparency.
It was being tested at Swarthmore College some years back, according to NPR:
Everbach and Klem recommended using so-called “fritted” glass. It's etched with closely spaced rows of small circles. When standing right in front of it, the glass is hard to see through, but Everbach says from a slight distance, the dots don't obscure vision that much.
(Entry drafted in 2006. I’m gonna try to post some of the draft entries that’ve been sitting around for a long time.)