My personal definition revolves around the time of 3:00 AM. That is, any flight that departs before 3:00 AM local and arrives after 3:00 AM local that is not so long that it could not cross the 3:00 AM mark during the flight. My rationale is that midnight is a good point to label "end of the day" and 6:00 AM is "beginning of day", so 3:00 AM is right in the middle.

B747-400's definition, while it may be the official one (defined by whom?), would include 10 PM to 11 PM or 10 PM to midnight flights, which I wouldn't think of as red-eye flights.