1) A group of compounds containing the porphin structure, four pyrrole rings connected by methine bridges in a cyclic configuration to which a variety of side chains are attached. The nature of the side chain is indicated by a prefix, as uroporphyrin, hematoporphyrin, etc. The porphyrins, in combination with iron, form the heme component in biologically significant compounds such as hemoglobin and myoglobin.

2) Any of various compounds with a structure that consists essentially of four pyrrole rings joined by four =CH- groups ; especially : one (as chlorophyll or hemoglobin) containing a central metal atom and usually having biological activity.