I'm aware of the distinction between ISO 8601's time zone designators (which are simply UTC offsets) and other much more complex notions of time zones such as the IANA tz database (which identifies time zones with an "Area/Location" string and contains information about each zone such as daylight saving time rules and even the historical changes in such rules).
That said, "time zone designator" is the formal name of the UTC offset in ISO 8601, and it's reasonable to refer to it as such whenever the distinction is clear.