The title of this question is about how being ignorant of the law is not an excuse for violating it. However, I believe being ignorant of the law is sometimes applicable as a defense. I am curious what the opposite term is (assuming my assumption isn't incorrect) And if my assumption is incorrect, would be interested in an elaboration. Furthermore, would be interested (if my assumption is correct) what the parameters for such a defense would actually be. (how could I go about arguing my innocense based on ignorance?)
The term doesn’t come up
Because, AFAIK, there are no circumstances where being ignorant of the law allow a person to escape culpability.
There are, however, laws that allow ignorance of the facts to be an excuse. For example, a person who receives stolen goods where it is reasonable to believe that they aren’t is not guilty of the crime (although the still don’t own the goods).
The closest term would seem to be mens rea ("guilty mind"), sometimes called "criminal intent". The general idea is that as part of establishing that someone has committed a crime, the prosecution must prove that the person possessed the state of mind required under statute for conviction. Note, however, that this does not mean the person needs to know that their conduct was against the law, but that they competently understood the circumstances of their actions which were against the law. Using Dale M's example of unknowingly receiving stolen goods, a person that receives goods but does not know they are stolen and is not negligent in their lack of knowledge (e.g. if the goods had some marking that would make it obvious the person in possession of them couldn't have bought them legally) may not possess the mens rea required to be convicted. Someone who is not mentally competent to stand trial would also lack mens rea. However, it should be noted again that none of this refers to the accused person's knowledge of the law, just their knowledge of the facts.