When the term yeast is used, most people think of freshly baked bread. Many people will also think of a cold, foamy headed beer. Both are made possible by yeast, but there are many more applications.
Yeast has been used to raise bread and make beer and wine since prehistory, and the work is very ancient. It comes to us in modern English via the Old English gyst, which in tern derived from the Indo-European word yes, meaning quite literally to bubble. Thus the word is very much older than our understanding that yeasts are living things, dating from the 1850s due to the work of Louis Pasteur.
When we think of yeast, we normally are referring to a single species (out of around 1500, give or take), Saccharomyces cerevisiae. This single species is responsible for raising bread, making wine and much of the beer that is drunk, as well as alcohol for beverage and industrial purposes. Unless I qualify, when I use the term “yeast” this is the species to which I refer.