The description of the word you requested from the astronomical dictionary is given below.
A redshift is a doppler shift, of characteristics (such as spectral
lines) in the frequency spectrum of light, to smaller frequencies, so
that yellow light gets more reddish. The opposite of redshift is
blueshift. The ratio of the frequencies of redshifted characteristics
and the frequencies of the unshifted characteristics is also called
redshift, usually denoted by a variable called
Systematic redshift is found in the spectra of all but the closest of galaxies, which is seen as evidence for the expansion of the Universe.