fermentation means: The chemical process whereby food undergoes a chemical reaction that is precipitated by enzymes from bacteria or yeast. It refers to the initial stages of rising after bread dough has been developed. This is where the dough gets leavened and the bread develops its flavor. (in Cooking Dictionary)

What else does fermentation mean?

The result of a leavening agent (e.g. yeast) is a breakdown in organic substances. (in Cooking Dictionary)

A group of chemical reactions that are triggered by enzymes or microorganisms. This includes the conversion of complex organic compounds to relatively simple substances and, in particular, yeast’s anaerobic sugar-to-carbon dioxide/alcohol. (in Merlin Dictionary)