Legal and formal term for a child born to unmarried parents. In everyday conversation, 非嫡出子 (ひちゃくしゅつし) or descriptive phrases are more common.
The legal status of an illegitimate child varies by country.
He was born an illegitimate child, but was later acknowledged by his father.
Historically referred to a child born to a concubine or a non-primary wife, carrying a different social nuance. 私生子 is a broader term for any child born out of wedlock.
私生子 combines 私生, birth outside a legally recognized marriage, with 子, “child.” It was formerly used as a legal and social classification.