The Collaborative International Dictionary of English v.0.48:

spinach beet \spinach beet\ n.
   1. a beet lacking a swollen root; it is grown as a vegetable
      for its edible leaves and stalks.

   Syn: chard, Swiss chard, spinach beet, leaf beet, chard
        plant, Beta vulgaris cicla.
        [WordNet 1.5] 2. a plant with long succulent whitish
        stalks with large green leaves.

   Syn: chard, Swiss chard, spinach beet, leaf beet.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

spinach beet
    n 1: beet lacking swollen root; grown as a vegetable for its
         edible leaves and stalks [syn: chard, Swiss chard,
         spinach beet, leaf beet, chard plant, Beta vulgaris
         cicla]
    2: long succulent whitish stalks with large green leaves [syn:
       chard, Swiss chard, spinach beet, leaf beet]