1. the status of a child whose parents were unmarried at the time of birth. In Western societies, the term has fallen into virtual disuse with changing family structures (e.g., cohabiting but unmarried parents, single mothers by choice), the fading of the stigma formerly attached to illegitimacy, and the disappearance of most legal distinctions between legitimate and illegitimate children.

2. the state of being unlawful, improper, or contrary to reason and logic. —illegitimate adj.