Antonyms for the term "more well-marked" could be "less obvious," "inconspicuous," "subtle," or "hidden." When something is "less obvious," it is not as easily noticeable or distinguished as something that is "more well-marked." Inconspicuous denotes something that is hidden or difficult to see and is completely opposite to "more well-marked." "Subtle" is used to describe things that are barely noticeable and do not attract attention, while "hidden" denotes something that is completely obscured or difficult to find. Therefore, if someone describes something as "less well-marked," it means that it is not as easily distinguished as something that is more well-defined.