a. Being against established or accepted rules and standards: an illegitimate means of winning a debate.

b. Being against the law; illegal: an illegitimate contract.

c. Not valid or defensible: Their reasons for missing school are illegitimate.

d. Incorrectly deduced; illogical: an illegitimate conclusion.

e. Biology Unacceptable as a scientific name because of not conforming to the international rules of nomenclature.