Of an orbit: A point in a body's orbit, defined from 0 to 360 degrees. For bodies orbiting the Sun, the 0 point of longitude is defined as Earth's vernal equinox.
On a body: the angular distance east to west from a standard meridian such as Earth's Greenwich. The poles of the body serve as north and south and lines running from north to south measuring the angular distances east and west are longitudinal lines.
Fact sheet: longitude used in the fact sheet pertains to the describing a point in a orbit. units are in degrees.