brewer's yeast

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various yeasts, especially Saccharomyces cerevisiae, used as a ferment in brewing and also as a source of B-complex vitamins.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any live yeast used in brewing or yeast obtained as a by-product of brewing, dried and killed, and used as nutritional yeast.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun yeast used in fermenting beer
• noun used as a leaven in baking and brewing