1) Chemical reaction 
Compare with chromophore. A group or substructure in a molecule that influences the intensity of absorption of the molecule.
An auxochrome (Gr. Auxanein:to increase, chroma:colour) is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light. They themselves fail to produce the colour; but when present along with the chromophores in an organic compound intensifies the colour of the chromogen. Examples include the hydroxyl ...
(from the article `colour`) ...the spectrum and therefore appear yellow or orange in colour. The same effect occurs if the number of electrons present on a conjugated chain is ...
a saturated group with nonbonded electrons which, when attached to a chromophore, alters both the wavelength and intensity of an absorption.
<biochemistry> This is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light. ... (09 Oct 1997) ... 
Type: Term Pronunciation: awk′sō-krōm Definitions: 1. The chemical group within a dye molecule by which the dye is bound to reactive end groups in tissues. The auxochrome enhances the intensity of absorption.
A group or substructure in a molecule that influences the intensity of absorption of the molecule. 
any radical or group of atoms that intensifies the color of a substance.