What is the X in X-mas?
Old English gist "yeast," common West Germanic (cf. Middle High German gest, German Gischt "foam, froth," Old High German jesan, German gären "to ferment"), from PIE *jes- "boil, foam, froth" (cf. Sanskrit yasyati "boils, seethes," Greek zein "to boil," Welsh ias "seething, foaming").
Any of various unicellular fungi of the genus Saccharomyces, especially S. cerevisiae, reproducing by budding and from ascospores and capable of fermenting carbohydrates.
Any of various similar fungi.
A commercial preparation in either powdered or compressed form containing yeast cells and inert material and used especially as a leavening agent or as a dietary supplement.
Any of various one-celled fungi that reproduce by budding and can cause the fermentation of carbohydrates, producing carbon dioxide and ethanol. There are some 600 known species of yeast, though they do not form a natural phylogenic group. Most yeasts are ascomycetes, but there are also yeast species among the basidiomycetes and zygomycetes. The budding processes in yeasts show a wide range of variations. In many yeasts, for example, the buds break away as diploid cells. Other yeasts reproduce asexually only after meiosis, and their haploid buds act as gametes that can combine to form a diploid cell, which functions as an ascus and undergoes meiosis to produce haploid spores. Still other yeasts form buds in both haploid and diploid phases. The ascomycete yeast Saccharomyces cerevisiae is used in baking to produce the carbon dioxide that leavens dough and batter. It has been the subject of extensive research in cell biology, and its genome was the first to be sequenced among eukaryotes. A variety of yeasts of the genus Saccharomyces are used in making beer and wine to provide alcohol content and flavor. Certain other yeasts, such as Candida albicans, are pathogenic in humans.