Other forms: rotundities
When you talk about the rotundity, or roundness, of a voice, you're talking about how full-bodied and easily heard it is. The rotundity of a person is how round they are in the belly. Any 3D object that has a roundness to it can be described in terms of its rotundity.
The word rotundity comes from the Latin rotundus meaning "round, circular, wheel-like." A rotunda is a circular building with a dome over it. The rotundity or roundness of its structure is what gives it its name. The rotundity of an opera singer's voice is what allows her song to carry and fill a room. The rotundity of Santa Claus' belly is part of what makes him the jolly character we love so much.