The Collaborative International Dictionary of English v.0.48:

Orach \Or"ach\, Orache \Or"ache\, n. [F. arroche, corrupted fr.
   L. atriplex, Gr. ?. Cf. Arrach.] (Bot.)
   A genus (Atriplex) of herbs or low shrubs of the Goosefoot
   family, most of them with a mealy surface.
   [1913 Webster]

   Garden orache, a plant (Atriplex hortensis), often used
      as a pot herb; -- also called mountain spinach.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Atriplex hortensis
    n 1: Asiatic plant resembling spinach often used as a potherb;
         naturalized in Europe and North America [syn: garden
         orache, mountain spinach, Atriplex hortensis]