porphyrogenitism
Por`phy*ro*gen"i*tism, n. Etym: [LL. porphyro genitus, fr. Gr. Defn: 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.