Marked means clearly noticeable or obvious. When there's a marked difference between two things, you can easily see how they're different. If a student shows marked improvement in math, everyone can tell they've gotten much better: the change is too big to miss.

The word suggests something that stands out or catches your attention. A marked contrast between a sunny morning and an afternoon thunderstorm is dramatic and striking. When a teacher notices marked enthusiasm in a usually quiet student, that excitement is unmistakable.

Sometimes marked carries a hint of concern or warning. A marked man is someone who has been singled out for attention, usually negative. In old adventure stories, when someone is “marked by enemies,” it means they have been targeted.

When something is marked, it has been set apart from everything else, made special or noticeable in some way, for better or worse.