What is active dry yeast?

The answer

Active dry yeast is baker's yeast, Saccharomyces cerevisiae, dried into small granules that keep for months before use. The cells are alive but dormant, and they wake up when they absorb lukewarm water, then feed on sugars and release carbon dioxide gas that makes dough rise. A 7 g (0.25 oz) packet holds about 2¼ teaspoons, according to Red Star Yeast. It differs from instant yeast mainly in its coarser granules, and from fresh yeast in being dry and shelf-stable.