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]

The Collaborative International Dictionary of English v.0.48:

Atriplex \Atriplex\ n.
   a genus of plants of the goosefoot family (Chenopodiaceae);
   its members include species called orach and saltbush.

   Syn: genus Atriplex.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

orach
    n 1: any of various herbaceous plants of the genus Atriplex that
         thrive in deserts and salt marshes [syn: orach, orache]