Source : Webster's Revised Unabridged Dictionary (1913)

Barm \Barm\, n. [OE. berme, AS. beorma; akin to Sw. b["a]rma, G.
   b["a]rme, and prob. L. fermenium. [root]93.]
   Foam rising upon beer, or other malt liquors, when
   fermenting, and used as leaven in making bread and in
   brewing; yeast. --Shak.

Barm \Barm\, n. [OE. bearm, berm, barm, AS. beorma; akin to E.
   bear to support.]
   The lap or bosom. [Obs.] --Chaucer.

Source : WordNet®

barm
     n : a commercial leavening agent containing yeast cells; used to
         raise the dough in making bread and for fermenting beer
         or whiskey [syn: {yeast}]