The Collaborative International Dictionary of English v.0.48:

Equisetum \Eq`ui*se"tum\, n.; pl. Equiseta. [L., the
   horsetail, fr. equus horse + seta a thick,, stiff hair,
   bristle.] (Bot.)
   A genus of vascular, cryptogamic, herbaceous plants; -- also
   called horsetails.
   [1913 Webster]

   Note: The Equiseta have hollow jointed stems and no true
         leaves. The cuticle often contains siliceous granules,
         so that one species (E. hyemale) is used for scouring
         and polishing, under the name of Dutch rush or
         scouring rush.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Equisetum
    n 1: horsetails; coextensive with the family Equisetaceae [syn:
         Equisetum, genus Equisetum]