A Crown Prince is the heir to the throne of a Kingdom, having either won Crown Tournament, or been the consort and inspiration of the winner of Crown Tournament.
Crown Princes are different from Territorial Princes, in that the latter hold lands in Fief from the crown. Both are considered royalty, though the Crown Prince is higher in precedence than a Territorial Prince.
Crown Princes exist to allow the heirs a sort of “training period” before ascending to the throne.