The Collaborative International Dictionary of English v.0.48:

Loosestrife \Loose"strife`\ (l[=oo]s"str[imac]f`), n. (Bot.)
   (a) The name of several species of plants of the genus
       Lysimachia, having small star-shaped flowers, usually
       of a yellow color.
   (b) Any species of the genus Lythrum, having purple, or, in
       some species, crimson flowers. --Gray.
       [1913 Webster]

   False loosestrife, a plant of the genus Ludwigia, which
      includes several species, most of which are found in the
      United States.

   Tufted loosestrife, the plant Lysimachia thyrsiflora,
      found in the northern parts of the United States and in
      Europe. --Gray.
      [1913 Webster]

WordNet (r) 3.0 (2006):

loosestrife
    n 1: any of numerous herbs and subshrubs of the genus Lythrum
    2: any of various herbs and subshrubs of the genus Lysimachia