porphyrogenitism

from The Century Dictionary.

• noun That principle of succession in royal families, especially in the families of the Byzantine emperors, in accordance with which a younger son, if born in the purple—that is, after the succession of his parents to the throne—was preferred to an older son who was not.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun rare 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.