From LASORS 2008, page 71:
For the purposes of this Schedule:
[...]
‘Cross-country flight’ means any flight during the course
of which the aircraft is more than 3 nautical miles from the
aerodrome of departure.
BUT... For the "experience requirements" of various licenses, ratings and so forth (eg. PPL, CPL, IR) this definition is normally augmented with a specific distance requirement.
So for PPL your XC needs to be 150 nm, with two intermediate stops, for CPL it needs to be 300 nm with two intermediate stops, and so forth.