porphyrogenitism

n. [ LL. porphyro genitus, fr. Gr. &unr_;; &unr_; 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. [ 1913 Webster ]