Difference between the terms illegal & unlawful
The terms illegal and unlawful have slight difference in meanings, as per Oxford English dictionary. But nowadays both terms are often used interchangeably.
Something that is Illegal is against the law in force such as illegal immigrants, whereas an unlawful act merely contravenes the rules that apply in a particular context such as an unlawful assembly. The term unlawful often implies an act that is done without lawful justification or authority.