What is the difference between ithm (sinfulness) and dhanb (sin), and what is the expiation for each?

There is no difference in meaning between "dhanb" (sin) and "ithm" (transgression), as both signify disobeying Allah's command or committing His prohibition, and both necessitate punishment.

"Dhanb" may be associated with "sayyi'ah" (evil deed), in which case it refers to a major sin, or it may stand alone, referring to any act of disobedience.

Similarly, "ithm" may be associated with "udwan" (aggression). In this context, "ithm" refers to what is forbidden by its very nature, while "udwan" refers to what is forbidden due to an excessive amount or transgression beyond what Allah has permitted. Alternatively, "ithm" may stand alone, encompassing "udwan" and referring to any act of disobedience.