Opaque set

In set theory and mathematics, an "opaque set" is not a standard or commonly used term. However, the concept of an opaque set might be used informally in certain contexts to refer to a set whose elements or the properties of which are not fully transparent or visible, or whose characteristics cannot be easily discerned. If you're encountering the term "opaque set" in a specific mathematical context, programming language, or another field, it may have a specialized meaning.