The Collaborative International Dictionary of English v.0.48:

Barm \Barm\ (b[aum]rm), n. [OE. berme, AS. beorma; akin to Sw.
   b[aum]rma, G. b[aum]rme, and prob. L. fermentum. [root]93.]
   Foam rising upon beer, or other malt liquors, when
   fermenting, and used as leaven in making bread and in
   brewing; yeast. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Barm \Barm\, n. [OE. bearm, berm, barm, AS. bearm; akin to E.
   bear to support.]
   The lap or bosom. [Obs.] --Chaucer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

barm
    n 1: a commercial leavening agent containing yeast cells; used
         to raise the dough in making bread and for fermenting beer
         or whiskey [syn: yeast, barm]