The Collaborative International Dictionary of English v.0.48:

Guelderrose' \Guel"der*rose'\, n. [Supposed to be brought from
   Guelderland; hence, D. Geldersche roos, G. Gelderische rose,
   F. rose de Gueldre, It. rose di Gueldra, Sp. rosa de
   Gueldres.] (Bot.)
   A cultivated variety of a species of Viburnum (V.
   Opulus), bearing large bunches of white flowers; -- called
   also snowball tree. Guelph

The Collaborative International Dictionary of English v.0.48:

Snowball \Snow"ball`\, n.
   1. A round mass of snow pressed or roller together, or
      anything resembling such a mass.
      [1913 Webster]

   2. (Bot.) The Guelder-rose.
      [1913 Webster]

   3. (Bot.) a shrub of the genus Viburnum, having large
      clusters of white flowers.
      [PJC]

   Snowball bush or Snowball tree (Bot.), the
      Guelder-rose.

   a snowball's chance in hell, [Colloq.] no chance; an
      infinitesimal chance.
      [1913 Webster +PJC]