light echo

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 such transients. Reflected light of a reflection nebula that shows no discernible variation wouldn't be termed a light echo.

Another term for transients, dust echo, indicates something similar to the light echo concept, but (by my reading) it is thermal emission produced by dust heated by the originating transient rather than actual reflection. (It wouldn't surprise me if the terms light echo and/or dust echo are not always distinguished: I believe the term reflection is sometimes used in a general way for various types of observable responses.)