What is the FAA definition of solo flight?
According to FAA, it means that the pilot should be the only person in the aircraft. According to 14 CFR § 61.51 - Pilot logbooks,


  (d) Logging of solo flight time. Except for a student pilot performing the duties of pilot in command of an airship requiring more than one pilot flight crewmember, a pilot may log as solo flight time only that flight time when the pilot is the sole occupant of the aircraft.


14 CFR § 61.8761.87 - Solo requirements for student pilots also says pretty much the same thing:


  (a) General. A student pilot may not operate an aircraft in solo flight unless that student has met the requirements of this section. The term “solo flight” as used in this subpart means that flight time during which a student pilot is the sole occupant of the aircraft or that flight time during which the student performs the duties of a pilot in command of a gas balloon or an airship requiring more than one pilot flight crewmember.