Short version: Which term generalizes "polyhedron" to include shapes whose faces are not necessarily flat?
The finite volume method is not very restrictive when it comes to the shape of the grid cells. OpenFOAM (a library that implements FVM), for example, requires that a cell be "contiguous, convex and closed" and defined as "a list of faces". A face is defined by a "list of points such that each two neighbouring points are connected by an edge"; the face center "needs to be inside the face", and not "all points of the face need to be coplanar".
First I thought of describing the class of allowed shapes for a cell as "polyhedra". However, a polyhedron must have flat faces, which is not a requirement for the cell. I wonder if "warped polyhedra" would better describe the shape or if there is an appropriate geometrical term for that class.