first-born imperial prince
Archaic term used in historical contexts for the eldest son of an emperor.
In old records, 一の御子 refers to the emperor's eldest son.
Compound of 一 (ichi, 'first') + の (no, possessive particle) + 御子 (miko, 'imperial child'). The exact historical derivation is uncertain, but the term follows a pattern of numbering imperial princes.