The Collaborative International Dictionary of English v.0.48:

Spinach \Spin"ach\, Spinage \Spin"age\, n. [OF. espinache,
   espinoche, F. ['e]pinard; cf. F. spinace, Sp. espinaca; all
   fr. Ar. isf[=a]n[=a]j, isfin[=a]j, aspan[=a]kh, probably of
   Persian origin.] (Bot.)
   A common pot herb (Spinacia oleracea) belonging to the
   Goosefoot family.
   [1913 Webster]

   Mountain spinach. See Garden orache, under Orache.

   New Zealand spinach (Bot.), a coarse herb (Tetragonia
      expansa), a poor substitute for spinach.
      [1913 Webster]

   Note: Various other pot herbs are locally called spinach.
         [1913 Webster]

WordNet (r) 3.0 (2006):

spinach
    n 1: southwestern Asian plant widely cultivated for its
         succulent edible dark green leaves [syn: spinach,
         spinach plant, prickly-seeded spinach, Spinacia
         oleracea]
    2: dark green leaves; eaten cooked or raw in salads