'Cross-country flight' means any flight during the course of which the aircraft is more than 3 nautical miles from the aerodrome of departure."
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This definition taken from the ANO is used to define "cross country" for the purposes of the privileges of a Flight Instructor. A FI(R), formerly (AFI) may not authorise a student on their first solo flight or first solo "cross country" defined as any flight more than three miles from the airfield.
For the purpose of claiming cross country hours towards licence issue (CPL etc) Cross country is any flight A to B or any flight from A to A with defined turning points. i.e. it is planned rather than a wander arround.