secular

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 also meant to imply a (long-term) non-periodic change, to distinguish it from an ongoing (shorter-term) periodic change. 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:

• secular variation or secular motion
• secular evolution (phrase seen regarding slow changes in galaxies).
• "secular phenomena"
• "secular perturbation"
• "secular chaos"
• "secular acceleration"
• "secular drift"

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