Illegal, a.
Etymology: [Pref. il- not + legal: cf. F. illégal.]
Not according to, or authorized by, law; specif., contrary to, or in violation of, human law; unlawful; illicit; hence, immoral; as, an illegal act; illegal trade; illegal love. Bp. Burnet.
Shades of meaning
Fernald has no entry of its own for illegal; he lists it under criminal.
Where he writes about it:
Every criminal act is illegal or unlawful, but illegal or unlawful acts may not be criminal. — from criminal
Offenses against public law are criminal; offenses against private rights are merely illegal or unlawful. — from criminal
It is illegal for a man to trespass on another's land, but it is not criminal; the trespasser is liable to a civil suit for damages, but not to indictment, fine, or imprisonment. — from criminal
Vicious refers to the indulgence of evil appetites, habits, or passions; vicious acts are not necessarily criminal, or even illegal; we speak of a vicious horse. — from criminal
Ingratitude is sinful, hypocrisy is wicked, but neither is punishable by human law; hence, neither is criminal or illegal. — from criminal