Legitimacy, in traditional Western common law, is the status of a child born to parents who are legally married to each other, and of a child conceived before the parents obtain a legal divorce. Conversely, illegitimacy, also known as bastardy, has been the status of a child born outside marriage, such a child being known as a bastard, a love child, a natural child, or illegitimate. In Scots law, the terms natural son and natural daughter bear the same implications.

The importance of legitimacy has decreased substantially in Western countries since the sexual revolution of the 1960s and 1970s and the declining influence of conservative Christian churches in family and social life.

Births outside marriage now represent a large majority in many countries of Western Europe and the Americas, as well as in many former European colonies. In many Western-influenced cultures, stigma based on parents' marital status, and use of the word bastard, are now widely considered dated.