Depends on the country. In UK they're all tights I belive. But in US, non-tranaparent are tights, otherwise they're pantyhose. Such as a ballerina wears tights, but the dark sheer hosery is pantyhose. Correct me if I'm wrong (which wouldn't surprise, I'm not a girl).
It also has to do with the fabric they're made out of. Pantyhose tends to be chemically treated nylon. tights tend to be sturdier materials such as spandex or cotton.