From Old English gist 'foam, froth,' from PIE *yes- 'to boil, bubble' — named for the foaming action, not the organism.
A microscopic fungus used as a leavening agent in baking and fermenting alcoholic beverages.
From Old English gist, gyst 'yeast, froth,' from Proto-Germanic *jestuz, from PIE *yes- 'to boil, foam, bubble.' The word originally described the foaming, bubbling action of fermentation rather than the organism itself — the fungal nature of yeast was not understood until Louis Pasteur's work in the 1860s. Key roots: *yes- (Proto-Indo-European: "to boil, foam").