Is there a name for the class of polyhedra that are (a) convex, and (b) have regular polygons for faces? I don't want to invent a name if a name already exists.
Call the class P. P would include,
the 'cap' of an icosahedron, i.e. the 5 triangles round a vertex plus their pentagonal base,
the rest of the icosahedron after removal of the cap,
the so called "siamese" dodecahedron,
a polyhedron of 3 pentagons and 5 (equilateral) trianges. It has one triangle connected to three pentagons, so the vertices there are (triangle-pentagon-pentagon), and there is then room for four triangles to fill the space left at the other end.
plus of course all the Platonic, Archimedean and prism/antiprism polyhedra.
A very simple idea, but I can't find on the web a word in use for the set P.
I do not think that there is a universally used and understood term for this class of polyhedra. That is, this particular class of polyhera does not have a name which is generally used in the literature. In my own writing, I would probably spell it out—this kind of object is a convex polyhedron with regular faces (CPRF, for short).
However, I believe that "semiregular polyhedron" is close, as long as a definition or some explanation is given. From the above linked Wikipedia page, a semiregular polyhedron is
which is not quite what the question asks for (as this definition imposes an additional condition on the configuration of vertices), but this seems reasonably close, and the term "uniform polyhedron" is more commonly used for vertex transitive polyhedra with regular faces.
For what it is worth, there is an interesting discussion of these kinds of polyhedra in
Indeed, as pointed out in a comment, some authors refer to such objects as "Johnson solids", likely due to the above-cited paper.