Secular, as used in astronomy, indicates occurrence over a long period of time and not periodic. It is especially used to discuss the long-term trends in a motion that is periodic over the shorter term, such as stating that/how an orbit changes over time.

The term is commonly used in describing dynamics and orbits. Some phrases:

• "secular variation"
• "secular phenomena"
• "secular perturbation"
• "secular chaos"
• "secular acceleration"
• "secular drift"

The term is also used in Time Series Analysis, with an analogous meaning.