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(occurring slowly over a long period of time)

The term secular, as used in astronomy, indicates changes happening very slowly, i.e., the change takes a long period of time. It is used to distinguish such a change from more rapid changes occurring to the same object/system. Sometimes it is meant to imply a non-periodic motion, to distinguish it from a simultaneous periodic motion. The term is often used in the kinematics of planets and their orbits: the Earth orbits the Sun very regularly, but over thousands or millions of years, there are changes in its orbit, termed secular. Some phrases:

The term is also used in time series analysis, with an analogous meaning.

Further reading:

Referenced by pages:
celestial mechanics
gravitational instability (GI)
Jacobi integral
Laplace-Lagrange secular theory
orbital resonance
perturbation theory