adj

• (Christianity) Not bound by the vows of a monastic order.
  "secular clergy in Catholicism"
• (literary) Centuries-old, ancient.
  "The long reaches that were like one and the same reach, monotonous bends that were exactly alike, slipped past the steamer with their multitude of secular trees looking patiently after this grimy fragment of another world, the forerunner of change, of conquest, of trade, of massacres, of blessings."
• (astrophysics, geology) Relating to long-term non-periodic irregularities, especially in planetary motion or magnetic field.
  "Laplace (1749–1827) "saved the world" by using probability theory to estimate the parameters accurately enough to show that the drift of Jupiter was not secular after all; the observations at hand had covered only a fraction of a cycle of an oscillation with a period of about 880 years."
• (atomic physics) Unperturbed over time.
  "The secular A and nonsecular B parts of hyperfine interaction for any particular frequencies ν_α and ν_β are derived from eqn.(21) by ..."