The purpose of the term alphanumeric isn't really to say that something contains letters and numbers; rather, it's to say that something contains nothing but letters and numbers. In fact, a string that contains only letters, or only digits, is still considered "alphanumeric", in the sense that it can appear in a field that accepts alphanumeric strings.
If you also want to allow "symbols and special characters", then the question is what you don't want to allow. If you mean that all characters are allowed, then it doesn't make sense to use a term like alphanumeric, because you'd need a term meaning "only anything", which is practically a contradiction.
If you don't want to allow control characters, then you can potentially use graphic or printable; but you'll need to rephrase a bit, because although we describe certain characters as "graphic" or "printable", we don't generally use those terms to describe strings that contain only those characters.