What's the name of a black disc or circle an animator uses to hold their drawing in place?

I saw an animator at Epcot use one of these things.

It was black and could swivel and move depending on what angle he wanted to use. It had a square of clear glass or plastic in the middle.

It was the same purpose of using a T-square on a draft table. A tool to hold the drawing in place on a draft table.

For now I use a strong tape to hold my drawings. in place on my draft table.