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).