An auxochrome can be defined as any group which does not itself act as a chromophore but whose presence brings about a shift of the absorption band towards the red end of the spectrum (longer wavelength). The absorption at longer wavelength is due to the combination of a chromophore and an auxo- chrome to give rise to another chromophore. An auxochromic group is called colour enhancing group.
Auxochromic groups do not show characteristic absorption above 200 mμ. Some common auxochromic groups are -OH, ―OR, -NH2, -NHR, -NR2, -SH etc. The effect of the auxo- chrome is due to its ability to extend the conjugation of a chromophore by the sharing of non-bonding electrons. Thus, a new chromophore results which has a different value of the absorption maximum as well as the extinction coefficient.
For example, benzene shows an absorption maximum at 255 mμ [εmax 203] whereas aniline absorbs at 280 mμ [εmax 1430]. Hence, amino (-NH2) group is an auxochrome.
Source: Elementary Organic Spectroscopy: Y.R.Sharma