Uncontrolled airspace is airspace where an Air Traffic Control (ATC) service is not deemed necessary or cannot be provided for practical reasons. According to the airspace classes set by ICAO both class F and class G airspace are uncontrolled. It is the opposite of controlled airspace.
ATC does not exercise any executive authority in uncontrolled airspace, but may provide basic information services to aircraft in radio contact. Flight in uncontrolled airspace will typically be under VFR. Aircraft operating under IFR should not expect separation from other traffic: however in certain uncontrolled airspace this might be provided on an 'as far as is practical' advisory basis.