If you look at the latest ANO you will see that this definition is no longer in the Interpretation section because it has being incorrectly interpreted. The definition is solely for the purpose of defining a "Cross Country flight" for the purposes of the limitations on a FI(R) or AFI rating. It has nothing to do with cross country time for a CPL!
ANO Schedule 7 Part B Ratings:
5. For the purposes of this Section—
“Cross-country flight” means any flight during the course of which the aircraft is more than three nautical miles from the aerodrome of departure.
which means that a FI(R) or AFI cannot authorise the first flight that lgoes more than 3nm from the aerodrome.
A Cross Country flight is generally recognised as any flight that goes from A to B or any flight with planned turning points.It is not defined anywhere.