An Etymological Dictionary of Astronomy and Astrophysics

An ordinary electromagnetic wave that goes forward with time. → Maxwell’s equations are indifferent to the distinction between past and future. It is therefore permissible for the electromagnetic waves to go backward in time. Forward-in-time waves are called retarded, as they arrive after they are sent by the transmitter. Backward-in-time waves are called → advanced wave.

See also: Retarded, adj. of → retard; → wavel.