The Collaborative International Dictionary of English v.0.48:

Bulrush \Bul"rush`\ (b[.u]l"r[u^]sh`), n. [OE. bulrysche,
   bolroysche; of uncertain origin, perh. fr. bole stem + rush.]
   (Bot.)
   A kind of large rush, growing in wet land or in water.
   [1913 Webster]

   Note: The name bulrush is applied in England especially to
         the cat-tail (Typha latifolia and Typha
         angustifolia) and to the lake club-rush (Scirpus
         lacustris); in America, to the Juncus effusus, and
         also to species of Scirpus or club-rush.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Juncus effusus
    n 1: tall rush with soft erect or arching stems found in
         Eurasia, Australia, New Zealand, and common in North
         America [syn: bulrush, bullrush, common rush, soft
         rush, Juncus effusus]