Common loanword for yeast, used in baking and brewing contexts. Synonymous with 酵母 (こうぼ), but イースト is more common in everyday cooking.
See also: 酵母
You need yeast to make bread.
This recipe uses dry yeast.
Add yeast to the bread dough and mix well.
Leave the dough in a warm place to let the yeast ferment.
Make the bread rise with yeast.