any of various single-celled fungi that reproduce asexually by budding or division
"The baker added active yeast to the dough, relying on these single-celled fungi to multiply through budding and make the bread rise."
An often humid, yellowish froth produced by fermenting malt worts, and used to brew beer, leaven bread, and also used in certain medicines.
"The brewer carefully stirred the yeast into the warm wort to begin the fermentation process for making ale."
In plain English: Yeast is a tiny living organism that makes dough rise and helps make beer and wine by eating sugar and releasing gas.
"The baker added yeast to the dough so that it would rise properly."
Usage: Yeast is the living microorganism added to dough or wort that causes fermentation, resulting in rising bread or carbonated beverages. Do not confuse this ingredient with yeast extract, which is a flavoring additive derived from dead yeast cells rather than an active leavening agent.
To ferment.
"The baker added warm water and sugar to activate the yeast so it could begin to ferment the dough."
The word yeast comes from Old English ġist, which traces back to Proto-Germanic *jestuz. Originally, it referred to the substance used in baking and brewing.