If you are working on a certificate or rating OTHER THAN ATP and are not military then cross country will be that flight time that includes a landing at an airport over 50NM away form the
original point of departure. Take off, fly to an airport 50 NM away, do a touch and go, continue sight seeing locally for 5 more hours before parking the plane. That whole flight legally counts because it fits the definition. You might call it abusive but it counts.
If you are working on an ATP certificate you can include all time during those flight that took you more than 50 NM straight line distance from original point of departure even if the flight does not include a landing more than 50NM away. Military bombers take off and land at the same place but sure do travel more than 50 miles during the flight. Military folks working on a commercial certificate can use this definition as well.
Now, for 135. The XC experience required to meet the minimums to qualify for PIC under part 135 only require a flight to include a landing at an airport other than the original point of departure. Take off, fly to the next closest airport that might be 5 miles away, put your wheels on that runway, do lazy ocho's until you are almost out of gas and then land. The WHOLE FLIGHT counts as XC for the purposes of 135 experience requirement.
DO NOT HAVE YOUR STUDENTS TAKE YOU TO AN AIRPORT 50 MILES AWAY ON EVERY LESSON
If you are working towards an ATP certificate then
I took the liberty to simplify (slightly) this text taken from FAR 61.1 Definitions. This will only apply to fixed wing pilots (other than recreational) because I removed all the notes and such about other stuff that probably does not apply to this crowd. If it does apply to you then go look it up yourself. Show of hands, who is still reading?
(3)
Cross-country time means—
(i) time acquired during flight—
(A) Conducted by a person who holds a pilot certificate;
(B) Conducted in an aircraft;
(C)
That includes a landing at a point other than the point of departure; and
(D) That involves the use of dead reckoning, pilotage, electronic navigation aids, radio aids, or other navigation systems to navigate to the landing point.
EXCEPTION
(ii) For the purpose of meeting the aeronautical experience requirements for a private pilot certificate, a commercial pilot certificate, or an instrument rating, time acquired during a flight—
(A) Conducted in an appropriate aircraft;
(B)
That includes a point of landing that was at least a straight-line distance of more than 50 nautical miles from the original point of departure; and
(C) That involves the use of dead reckoning, pilotage, electronic navigation aids, radio aids, or other navigation systems to navigate to the landing point.
(iii) For the purpose of meeting the aeronautical experience requirements for a sport pilot certificate
...omited
(iv) For the purpose of meeting the aeronautical experience requirements for a sport pilot certificate with powered parachute privileges
...omited
(v) For the purpose of meeting the aeronautical experience requirements for any pilot certificate with a rotorcraft
...omited
(vi) For the purpose of meeting the aeronautical experience requirements for an airline transport pilot
(ATP) certificate time acquired during a flight—
(A) Conducted in an appropriate aircraft;
(B)
That is at least a straight-line distance of more than 50 nautical miles from the original point of departure; and
(C) That involves the use of dead reckoning, pilotage, electronic navigation aids, radio aids, or other navigation systems.
(vii) For a military pilot who qualifies for a commercial pilot certificate (except with a rotorcraft category rating) under §61.73 of this part, time acquired during a flight—
(A) Conducted in an appropriate aircraft;
(B) That is at least a straight-line distance of more than 50 nautical miles from the original point of departure; and
(C) That involves the use of dead reckoning, pilotage, electronic navigation aids, radio aids, or other navigation systems.