The Collaborative International Dictionary of English v.0.48:

Herb \Herb\ ([~e]rb or h[~e]rb; 277), n. [OE. herbe, erbe, OF.
   herbe, erbe, F. herbe, L. herba; perh. akin to Gr. forbh`
   food, pasture, fe`rbein to feed.]
   1. A plant whose stem does not become woody and permanent,
      but dies, at least down to the ground, after flowering.
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   Note: Annual herbs live but one season; biennial herbs flower
         the second season, and then die; perennial herbs
         produce new stems year after year.
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   2. Grass; herbage.
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            And flocks
            Grazing the tender herb.              --Milton.
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   Herb bennet. (Bot.) See Bennet.

   Herb Christopher (Bot.), an herb (Act[ae]a spicata),
      whose root is used in nervous diseases; the baneberry. The
      name is occasionally given to other plants, as the royal
      fern, the wood betony, etc.

   Herb Gerard (Bot.), the goutweed; -- so called in honor of
      St. Gerard, who used to be invoked against the gout. --Dr.
      Prior.

   Herb grace, or Herb of grace. (Bot.) See Rue.

   Herb Margaret (Bot.), the daisy. See Marguerite.

   Herb Paris (Bot.), an Old World plant related to the
      trillium (Paris quadrifolia), commonly reputed
      poisonous.

   Herb Robert (Bot.), a species of Geranium (Geranium
      Robertianum.)
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WordNet (r) 3.0 (2006):

herb robert
    n 1: a sticky low herb with small reddish-purple flowers;
         widespread in the northern hemisphere [syn: herb robert,
         herbs robert, herb roberts, Geranium robertianum]