Hexahydroporphyrins in which the nitrogen atoms and four
meso positions are saturated.
Sources:
PAC, 1987, 59, 779. 'Nomenclature of tetrapyrroles (Recommendations 1986)' on page 798 (https://doi.org/10.1351/pac198759060779)
White Book, 2nd ed., p. 294 (https://iupac.qmul.ac.uk/bibliog/white.html)