Especially pertaining to animal bodies, indicating the presence of a top and bottom but no distinctive differentiation of sides as considered from any given direction.
Picture in particular a spinning , which looks different from above versus below but not from any given "side". Most animals possessing radial symmetry can be described as being members of what can be described as the Radiata. Contrast with bilateral symmetry as well as the Bilateria.
Radially symmetrical animals include members of phylum Cnidaria and phylum Ctenophora. In a derived form, members of phylum Echinodermata also display an approximation of radial symmetry. These include many sessile or, at least, slow moving animals.
Note that a sponge (parazoan) is not considered to display symmetry so therefore is not considered to display radial symmetry and this is even though often can look as though they are radially symmetrical.
One can picture a transition from radial symmetry to bilateral symmetry that begins with cephalization (the mouth end) along with an elongation of a body (picture a with anterior and posterior but neither dorsal nor ventral sides). This would be rather that starting with formation of a left and a right and then developing cephalizsation. A tube, that is, is radially symmetrical.