A tornado is "a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud visible as a funnel cloud" For a vortex to be classified as a tornado, it must be in contact with both the ground and the cloud base. Definitions of Tornado refers to the vortex of wind, not the condensation cloud.
A tornado is not necessarily visible however, the intense low pressure caused by the high wind speeds and rapid rotation (due to cyclostrophic balance ) usually causes water vapor in the air to condense into cloud droplets due toThis results in the formation of a visible funnel cloud or condensation funnel.