1. Not according to law; not regular or authorised; unlawful; improper.
2. Unlawfully begotten; born out of wedlock; bastard; as, an illegitimate child.
3. Not legitimately deduced or inferred; illogical; as, an illegitimate inference.
4. Not authorised by good usage; not genuine; spurious; as, an illegitimate word.
5. (Science: botany) illegitimate fertilization, or illegitimate union, the fertilization of pistils by stamens not of their own length, in heterogonously dimorphic and trimorphic flowers.