auxochrome

from The Century Dictionary.

• noun In chem., noting those groups contained in dyes which enable them to combine with the fiber of the cloth or with a mordant. The most important of these are the hydroxyl-, amino-, sulpho-, and carboxyl-groups.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun obsolete ,(chemistry) any atom or group which, when added to a chromophore, causes a bathochromic shift in its spectrum