A cross country in FAA speak is landing at any airport other than the point of departure. No minimum distance is specified.
However, you need to have logged certain amounts of cross country time as a requirement for testing for ratings. The FARs require these cross countrys to be a minimum of 50nm straight line. In practice, this means you only log as cross country those flights which are over 50nm straight line between airports of arrival and departure. Once you have collected all your ratings you don't even need to do that (in fact you don't need to once you have logged the required time anyway)
For those of you who collect strange exceptions, there is an exception for military pilots who can take a load of bombs half way around the world, release them and go back to the airport of departure without landing elsewhere in between. They can log that as cross country if they wish.