Something that is secular happens gradually or takes place over a long period. Secular change is a continuous, nonperiodic change in one of the attributes of the states of a system. Secular perturbation is a slow, continuous change in one of the elements of an orbit. Secular stability is a condition in which the equilibrium configuration of a system is stable over a long period of time. A secular variable is a star whose brightness changes over centuries.
Secular acceleration is the apparent acceleration of the Moon and Sun across the sky, caused by an extremely gradual reduction in Earth's rotation rate (one 50-millionth of a second per day).
Secular parallax is the angle subtended at a star by a baseline that is the distance the Sun moves in a given interval of time with respect to the local standard of rest (4.09 AU per year).