As the consultation of some 20 monolingual dictionaries of the English language, a few technical dictionaries of law and commercial law, dictionaries of collocations and thesauri has established that the denotations (semantics) of the two adjectives do not deviate very much from each other. The two dictionaries that best render the semantic differences are the Oxford Dictionary of English (ODE), the Oxford American Dictionary (OAD), and Webster`s Third New International Dictionary WTNID (latest editions each).
OED/OAD:
entry: illegal: contrary to or forbidden by law, especially criminal law
entry: unlawful: not conforming to, permitted by, or recognized by law or rules
The second part of the upper definition is a first hint to the difference between the two terms.
In addition to its definitions the OED offers a usage note to explicate the differences. It says:
Illegal und unlawful have slightly different meanings, although they are often used interchangeably. Something that is illegal is against the law, whereas an unlawful act merely contravenes the rules that apply in a particular context. Thus handball in soccer is unlawful, but it is not illegal.
The non-criminal meaning of unlawful is also emphasized by the third and fourth senses in the WTNID entry unlawful:
3 : contrary to normal or acceptable procedure : IRREGULAR; esp : not morally right or conventional (unlawful love) (unlawful pleasures)
4 : born out of wedlock : ILLEGITIMATE
More confirming evidence can be obtained from the Specialist Dictionary of Law (Pons)
Illegal: not legal or against criminal law
Unlawful: (act) which is against the law
It may shed light on the matter to cite some collocations with the two adjectives taken from law dictionaries:
illegal: act, activities, conditions, considerations, contract, interest, practices, profit, trade, transactions, carrying of arms, immigrants, drugs
unlawful: act, assembly, detainer, picketing, profits, trespass on property, sexual intercourse, assembly, ban, violence
That wraps it up for tonight!
appendix for fivejedjon (and anyone interested):
The following two para are quotations from your no. 7 (my emphasis)
Although married couples were lawfully wedded, I have not heard of an illegitimate child (i.e. one born to an unmarried couple) referred to as an unlawful child.
ad 1) comp. WTNID no. 4!
Equally, I have not heard of a love affair called unlawful and legal; It may be illicit, but I don't think it's unlawful unless it's illegal.
ad 2) love affair called unlawful and legal - comp. WTNID no. 3 (moral meaning) and collocations!
ad 3) unless it's illegal - here is the definition of unlawful sexual intercourse (quotation from: Pons Law Dictionary): sexual intercourse with someone under the age of consent
P.S. What unheard-of argument is that: “I have not heard …” Most existing things have never been heard by you or anyone!
