The Collaborative International Dictionary of English v.0.48:

Sorrel \Sor"rel\, n. [F. surelle, fr. sur sour, fr. OHG. s?r
   sour. See Sour.] (Bot.)
   One of various plants having a sour juice; especially, a
   plant of the genus Rumex, as Rumex Acetosa, Rumex
   Acetosella, etc.
   [1913 Webster]

   Mountain sorrel. (Bot.) See under Mountain.

   Red sorrel. (Bot.)
   (a) A malvaceous plant (Hibiscus Sabdariffa) whose acid
       calyxes and capsules are used in the West Indies for
       making tarts and acid drinks.
   (b) A troublesome weed (Rumex Acetosella), also called
       sheep sorrel.

   Salt of sorrel (Chem.), binoxalate of potassa; -- so called
      because obtained from the juice of Rumex Acetosella, or
      Rumex Axetosa.

   Sorrel tree (Bot.), a small ericaceous tree (Oxydendrum
      arboreum) whose leaves resemble those of the peach and
      have a sour taste. It is common along the Alleghanies.
      Called also sourwood.

   Wood sorrel (Bot.), any plant of the genus Oxalis.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sorrel tree
    n 1: deciduous shrubby tree of eastern North America having
         deeply fissured bark and sprays of small fragrant white
         flowers and sour-tasting leaves [syn: sorrel tree,
         sourwood, titi, Oxydendrum arboreum]
    2: Australian tree with acid foliage [syn: sorrel tree,
       Hibiscus heterophyllus]