What is "porphyrogenitism"

Porphyrogenitism \Por`phy*ro*gen"i*tism\, n. [LL. porphyro genitus, fr. Gr. ?; ? purple + root of gi`gnesqai to be born.] The principle of succession in royal families, especially among the Eastern Roman emperors, by which a younger son, if born after the accession of his father to the throne, was preferred to an elder son who was not so born.
--Sir T. Palgrave.

n. (context rare English) The principle of succession in royal families, especially among the Eastern Roman emperors, by which a younger son, if born after the accession of his father to the throne, was preferred to an elder son who was not so born.