Natural pigments containing four pyrrole rings joined by one-carbon units linking position 2 of one pyrrole ring to position 5 of the next. Porphyrins are macrocyclic tetrapyrroles. E.g. bilin (a linear tetrapyrrole).
PAC, 1995, 67, 1307 (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1371
See also:
White Book, p. 279