A light echo is reflected light from an astronomical event that, due to its longer path, arrives measurably later than electromagnetic radiation directly from the event. An example is the reflection of a supernova's light on a cloud of dust. Depending upon the distance and angles between the EMR source and the reflecting entity, the delay can be any amount of time from seconds to years or more. If the reflecting entity is in front of but slightly to the side of the source, i.e., a "glancing" reflection, the view from Earth can give the appearance of superluminal motion.
I believe the term light echo is generally used for transients. Reflection nebulae show reflected light but not necessarily showing discernible variation.