Keeping it super simple, Part 61 recognizes three distinct types of cross country flights. Which type you use depends on what you are going to use it for. I'll leave out the semi-obvious you gotta navigate there in some way:
- Landing at another airport > 50 NM away. Qualifies for all purposes; required to qualify for all Part 61 pilot certificates and ratings except ATP;
- >50 NM without a landing. A tip of the hat to our military flyers, but qualifies for ATP even if you were not in the military;
- Any distance with a landing at another airport. Basic definition; for the FAA, applies to such things as Part 135 pilot qualifications. Might apply to other things, many of it having nothing to do with the FAA.