What do you call a disk with a hole in the middle?

Solution 1:

There is an informal, nontechnical-English answer and a technical mathematical answer.

• informally, it can be a ring (like a coffee ring, aerobie, or washer (the last one is questionable, could be 'washer-shaped'), or a disk or disk with hole in it for compact disk (because the hole is somewhat secondary).

• technically, it is an annulus.

Solution 2:

In math it is called annulus. You also have the washer method, a method to calculate volumes using washers.

Edit:
On second thought, mathematically a "washer" is a 3D object.
It is worth mentioning Steven Pinker:

Few people think of a wire as a very, very thin skinny cylinder and of a CD as a very short one, though technically that's what they are. We conceive of them as having only one or two primary dimensions, respectively.