A SCOBY, or symbiotic culture of bacteria and yeast, is a population of single-celled organisms that is fundamental for many fermented foods, including sourdough, vinegar and kombucha. It is also commonly called "the mother" by brewers.
The term SCOBY is used primarily by kombucha brewers to refer to the cellulose mat that forms in the tea during the fermentation process, but the biochemical processes involved in sourdough are identical.