A bun foot is one of the more simple sorts of furniture feet you might see. While still clearly fancy and often paired with cabriole legs, they are a step down from a scroll foot or a claw and ball foot.
A bun foot is basically a lump. A nice, well curved, finished and polished lump, but the shape doesn't really suggest anything -- not even a bun, really. These are flattened spheroids, sometimes finishing off a gently curved leg with an ovoid swelling, sometimes looking very much like a chunky piece of furniture is just to heavy for the set of balls holding it up. Except, not even that. Some woodworkers will unabashedly use a 'square bun', basically a cube with sides tapering down and inwards in a curve. Some are simple pucks, while others are fluted or topped with cornices or other decorations.
The long and short of it is, if your foot is curvy and doesn't look much like anything in particular, you can get away with calling it a bun foot.