Apse line

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In astrodynamics apse line (or line of apsides) is a common name for an imaginary line defined by orbit's eccentricity vector. It is strictly defined for elliptic, parabolic, and hyperbolic orbits. For those orbits it can be imagined as a line:

between orbit's periapsis and apoapsis - for elliptical orbits; also known as the major axis
between orbit's periapsis and focus - for parabolic and hyperbolic orbits

Note: For circular orbits apse line is not defined as eccentricity is equal to zero. As it is required as a base for definition of true anomaly it is usually arbitrarily assumed (usually as a line pointing into direction of vernal equinox)