I believe they are usually called threadless adapters, since I'm not sure how (or why) you'd design an adapter to take a quill stem with a threadless headset.
A quill stem looks like this:
It slips inside the threaded headset (There are no threads on the stem itself, threadless refers only to the headset), then you tighten a bolt at the top to force the wedge against the headset, locking the stem in place.
An adapter looks like this:
The bit that goes inside the headset is the same as a normal quill stem, with a wedge. The top, instead of bending to a handlebar clamp, provides a threadless headtube-sized clamping surface for a threadless stem, like this:
