means A microscopic single-celled fungus that ferments sugars into carbon dioxide and alcohol, used to leaven bread and brew beer and wine.

from From Old English 'gist' or 'gyst,' tracing back to a Proto-Indo-European root meaning to boil, foam, or bubble up — the same churning idea behind Greek 'zein' (to boil) and likely a cousin of words for froth and effervescence. Long before anyone could see the organism through a lens, people named it for the only thing they could observe: the lively bubbling froth it raised in the dough and the vat.