1.NounA theoretical type of orbit which goes in a straight-line, up and down; with 100% orbital eccentricity (eccentricity=1.0); the orbital path passing through the center-of-mass of the primary mass being orbited; having an periapsis at 0, and two apoapsides, as both arms going out from the periastron are equidistant; where the orbital ellipse has zero width semi-minor axis/minor axis, and the path of the ellipse is the same as the major axis.