The Collaborative International Dictionary of English v.0.48:

Yerba \Yer"ba\, n. [Sp.] (Bot.)
   An herb; a plant.
   [1913 Webster]

   Note: This word is much used in compound names of plants in
         Spanish; as, yerba buena [Sp., a good herb], a name
         applied in Spain to several kinds of mint (Mentha
         sativa, Mentha viridis, etc.), but in California
         universally applied to a common, sweet-scented labiate
         plant (Micromeria Douglasii).
         [1913 Webster]

   Yerba dol osa. [Sp., herb of the she-bear.] A kind of
      buckthorn (Rhamnus Californica).

   Yerba mansa. [Sp., a mild herb, soft herb.] A plant
      (Anemopsis Californica) with a pungent, aromatic
      rootstock, used medicinally by the Mexicans and the
      Indians.

   Yerba reuma. [Cf. Sp. reuma rheum, rheumatism.] A low
      California undershrub (Frankenia grandifolia).
      [1913 Webster]

WordNet (r) 3.0 (2006):

Micromeria douglasii
    n 1: trailing perennial evergreen herb of northwestern United
         States with small white flowers; used medicinally [syn:
         yerba buena, Micromeria chamissonis, Micromeria
         douglasii, Satureja douglasii]