A shift of a spectral line to redder (longer) wavelengths. There are two types of redshift discussed in this course: (1) Doppler shift – resulting from motion of the emitting object away from the observer. This is the case for some nearby galaxies, but this is not the underlying physical cause of most cosmological redshifting. (2) Cosmological redshift – resulting from expansion of the Universe (see the Hubble constant, Big Bang) which stretches the wavelength of the light, making the light redder.