The Collaborative International Dictionary of English v.0.48:

Vervain \Ver"vain\, n. [OE. verveine, F. verveine, fr. L.
   verbena, pl. verbenae sacred boughs of laurel, olive, or
   myrtle, a class of plants; cf. verbenaca vervain. Cf.
   Verbena.] (Bot.)
   Any plant of the genus Verbena.
   [1913 Webster]

   Vervain mallow (Bot.), a species of mallow (Malva Alcea)
      with rose-colored flowers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vervain
    n 1: any of numerous tropical or subtropical American plants of
         the genus Verbena grown for their showy spikes of variously
         colored flowers [syn: verbena, vervain]