It is about any act that, in some way, breaks a rule or does serious harm to someone. The opposite of that term is lawful, a word that refers to the quality that a person or object has for not hiding anything or not having carried out an act of dubious ends. It is much more related to the legal field, since there it is used to label the series of acts committed by a criminal. Traditionally, it is seen as an aberration to the ethics, morals and principles of the person who commits them, so they automatically become a threat to society and must be locked up in jail, preventing them from continuing to commit crimes.
However, there are cases in which the nature of the term is specified with this term, such as illicit enrichment, an act in which a subject seeks to obtain economic benefits based on strategies that break some of the most important and well-known laws, such as theft, usury, fraud or tax fraud. These include profit directed at one or a group of people, who lose large amounts of money. Crimes of this nature are punishable quite easily, if they are brought to trial and concrete evidence is presented on the case.
In other, much simpler contexts, illicit acts can be all those actions that are marked for being carried out in secret and, if revealed, will hurt a person, such as extramarital affairs. In addition to this there are also the small out of place behaviors that are carried out in public, such as driving while intoxicated or running a traffic light, which are punished with some fines.