I always try to revert to official documents and what I found in ICAO Annex 1 - Flight Crew Licensing is the following:
Cross-country. A flight between a point of departure and a point of arrival following a pre-planned route using standard navigation procedures.
During ATPL training, I do not recall having seen a definition of cross-country flights, but in my carpels PPL courses I seem to remember the 40km (25NM) threshold. 2 remarks:
- Carpels is not an official source for definitions
- nevertheless, they can still be correct as the general principle is that ICAO states minimum requirements and any state can impose more severe/strict requirements as long as they do not conflict with the ICAO requirements.
An example of this is the transition altitude. According to ICAO, transition altitude should be at least 3.000' AMSL. The Belgian AIP promulgates 4.500' AMSL, which is more stringent and in respect of the ICAO stipulation.
Hope this already clarifies a little and will come back to you once I've checked Carpels.
Best regards