The Collaborative International Dictionary of English v.0.48:

Culver's physic \Cul"ver's phys"ic\, or Culver's root \Cul"ver's
root`\ [So called after a Dr. Culver, who used it.] (Bot.)
   The root of a handsome erect herb (Leptandra Virginica syn.
   Veronica Virginica) common in most moist woods of North
   America, used as an active cathartic and emetic; also, the
   plant itself.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Culvers root
    n 1: a tall perennial herb having spikes of small white or
         purple flowers; common in eastern North America [syn:
         Culver's root, Culvers root, Culver's physic,
         Culvers physic, whorlywort, Veronicastrum virginicum]