The Collaborative International Dictionary of English v.0.48:

cowherb \cowherb\ n.
   a European annual with pale rose-colored flowers; cultivated
   flower or self-sown grainfield weed; introduced in North
   America; sometimes classified as a soapwort; -- also called
   the cow-cockle.

   Syn: cow cockle, cow-cockle, Vaccaria hispanica, Vaccaria
        pyramidata, Saponaria vaccaria.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

cowherb
    n 1: European annual with pale rose-colored flowers; cultivated
         flower or self-sown grainfield weed; introduced in North
         America; sometimes classified as a soapwort [syn:
         cowherb, cow cockle, Vaccaria hispanica, Vaccaria
         pyramidata, Saponaria vaccaria]