Redshift refers to the difference in frequency of an electromagnetic wave as measured by a source versus a receiver in relative motion.
Redshift typically refers to the difference between the frequency of an electromagnetic wave as measured by its source and the frequency as measured by a receiver when the source and the receiver are in relative motion. When differentiated from "blueshift", it refers to a decrease in frequency, while "blueshift" refers to an increase in frequency.
The name "redshift" derives in the context of astronomy in comparison of spectral lines in the humanly visible range of electromagnetic waves; concerning distant (and, generally, receding) stars as senders, and human observers on Earth as receivers (with the sun as a reference star).
