Uncontrolled airspace is airspace

Airspace

Airspace means the portion of the atmosphere controlled by a country above its territory, including its territorial waters or, more generally, any specific three-dimensional portion of the atmosphere....

 where an Air Traffic Control

Air traffic control

Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...

 (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

Controlled airspace

Controlled airspace is an aviation term used to describe airspace in which ATChas the authority to control air traffic, the level of which varies with the different classes of airspace. Controlled airspace is established mainly for three different reasons:...

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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

Visual flight rules

Visual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minimums, as specified in the rules of the...

. 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.