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An **apse line**, or **line of apsides**, is an imaginary line defined by an orbit's eccentricity vector. It is strictly defined for elliptic, parabolic, and hyperbolic orbits.

For such orbits the apse line is found:^{[1]}

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

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

- Apsidal precession
- Apsis
- Eccentricity (orbit)
- Orbit: circular, elliptic, parabolic and hyperbolic
- True anomaly

**^**"Line of apsides – Astronomy".*Encyclopædia Britannica*. Retrieved 23 August 2015. CS1 maint: discouraged parameter (link)