For example, you're a fashion designer who wants to get the opinion of both a professional and someone who doesn't know much about fashion. What's a term for the latter person?
- A person without professional or specialized knowledge in a particular subject
If you must, you can say lay person.
"uneducated; non-clerical," early 14c., from Old French lai "secular, not of the clergy" (Modern French laïque), from Late Latin laicus, from Greek laikos "of the people," from laos "people," of unknown origin. In Middle English, contrasted with learned, a sense revived 1810 for "non-expert."
Layman - a person without professional or specialized knowledge in a particular subject.
It depends on how much not knowing:
- An 'amateur': they have some hobby-level knowledge.
- A 'novice': they have a small degree of professional knowledge. (Versus an expert or an adept, who would have a high level.)
- A 'lay person': they have no domain knowledge at all, and would need concepts translated to other terms. (Note this originally had a religious connotation: a priest would speak to God on behalf of lay persons, or the religious but not ordained people of the community.)
- A 'neophyte': A beginner in the domain, or a recent convert. The main point here is that they are recently of the domain. (Whereas someone might be a novice for a very long time.)
In the context of something like software the term would be user. Here customer would be better, particularly in a construction like customer-opinion.
Maybe not quite what you want but, framed as an amateur is: a Jack of all trades, master of none. It is vernacular.