Definitions
USMC:
Waterline(s): parallel lines of reference, in inches, more or less parallel to the bottom of the aircraft. The idea being that if the aircraft were placed in water, this would be the resulting line. There will be a Zero reference line; anything above is referenced with "+" so many inches, and those below are "-."
Station: a reference for distance from nose to tail, in inches. Zero is not necessarily on the aircraft...often in front of the nose.
Buttock Line: References left and right of the centerline, in inches.