biological parent; real parent
Refers to one's birth parent, as opposed to a stepparent, adoptive parent, or foster parent. Often used in contexts discussing family background or legal parentage.
He has never met his biological parents.
She is in contact with both her biological parents and her adoptive parents.
Compound of 実 (じつ, 'true, real') + の (possessive particle) + 親 (おや, 'parent'). The phrase literally means 'real parent' and is a straightforward modern expression.