noun
real surname; original family name
Refers to one's true or original family name, as opposed to an alias, pen name, or married name. Often used in historical or formal contexts.
彼はペンネームを使っているが、本姓は田中だ。
He uses a pen name, but his real surname is Tanaka.
結婚して姓が変わったが、本姓は山田です。
My surname changed when I got married, but my original family name is Yamada.