your heir; his heir
Honorific term used to refer respectfully to someone else's heir or successor. Not used for one's own heir.
令嗣のご活躍をお祈り申し上げます。
I wish your heir great success.
社長は令嗣に事業を譲る意向を示された
The president indicated his intention to hand over the business to his heir.
Composed of 令 (good, excellent, honorable) and 嗣 (heir, succeed). The exact historical derivation is uncertain, but the kanji combination conveys a respectful reference to an heir.