Yes. It's a hollow in a stone, usually hemispherical and artificial or at least enhanced. However, several perfectly natural solution pits in limestone boulders have been used in the same way as artificial bullauns, so it's a bit blurred.

It's more about the rites associated with the stone than the nature of the stone itself if you see what I mean. If it's been used as a bullaun stone then it's a bullaun stone.