A slimy, gummy substance made up of various bacteriaspecifically Mycoderma acetithat cause fermentation in wine and cider and turn them into vinegar. Known as mère de vinaigre in French and sometimes simply as "mother" in English, its growth is best fostered in a medium-warm environment (60°85°F). The mother should be transferred to a new mixture or discarded once the liquid has turned to vinegar.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.