Secular evolution is defined as slow, steady evolution. In galaxies, such evolution is either the result of long-term interactions between the galaxy and its environment (such as gas accretion or galaxy harassment), or it is induced by internal processes such as the actions of spiral arms or bars. Secular evolution therefore plays a important part in the formation of disk galaxies, with both the disk and bulge potentially involved, but is probably relatively unimportant in the formation of elliptical galaxies.
The most easily recognisable example of secular evolution in disk galaxies is the formation of stars in the spiral arms. This is induced by the action of the spiral structure on the disk of the galaxy. Although evidence for secular evolution in bulges is a little less clear, young stars have been found in the centres of many galaxies, including the Milky Way. One explanation is that gas has been funnelled into the galaxy centre (perhaps through the action of a bar) and a centrally concentrated burst of star formation has resulted. This is believed to be one of the mechanisms for creating starburst galaxies. Another secular evolution process associated with bars is the growth of bulges through kinematic disturbance. In this scenario, the galactic bar perturbs the central disk stars out of their regular orbits, either creating or expanding the bulge.