The term refers to "amateurs" in the sense that they did not study at a university or get a degree. AFAIU, the term "citizen" is preferred because "amateur" the

The term refers to "amateurs" in the sense that they did not study at a university or get a degree. AFAIU, the term "citizen" is preferred because "amateur" these days carries a connotation of "doesn't know what they're doing", while citizen scientists have made (and continue to make) meaningful contributions to astronomy. (Possibly other fields, too, but I don't know one way or the other.)