heir to the throne; successor to the throne
Formal term used in imperial contexts; refers to the person next in line to succeed the emperor.
The heir to the throne is determined according to the Imperial House Law.
The current heir to the throne is His Imperial Highness Crown Prince Akishino.
Compound of 皇位 (imperial throne) + 継承 (succession) + 者 (person). A straightforward modern term with no obscure historical derivation.