From Websters 1828 Dictionary (link)

Oleander OLEAN'DER, n. a plant of the genus Nerium the rose-bay or South sea rose; a beautiful shrub with flowers in clusters, of a fine purple color, but of an indifferent smell. The plant, especially the bark of the roots, is said to be poisonous.

From WordNet (r) 2.1 (2005) (link)

  oleander
      n 1: an ornamental but poisonous flowering shrub having narrow
           evergreen leaves and clusters of fragrant white to pink or
           red flowers: native to East Indies but widely cultivated in
           warm regions [syn: oleander, rose bay, Nerium
           oleander}]

From Merriam-Webster's Collegiate Dictionary, 11th Edition (2003) (link)

  oleander noun
   Etymology: Medieval Latin, alteration of arodandrum, lorandrum,
   perhaps alteration of Latin rhododendron — more at rhododendron
   Date: 1545
    a poisonous evergreen shrub (Nerium oleander) of the dogbane family
    with clusters of fragrant white to red flowers
  
  

From Oxford English Reference Dictionary (link)

oleander
n. an evergreen poisonous shrub, Nerium oleander, native to the Mediterranean and bearing clusters of white, pink, or red flowers.
Etymology: med.L

From Collins COBUILD Advanced Learner's English Dictionary (link)

  oleander
  (oleanders)
    An oleander is an evergreen tree or shrub that has white, pink, or purple
    flowers. Oleanders grow in Mediterranean countries and in some parts of Asia and Australia.
   N-VAR
  
  
  

From English Explanatory Dictionary (link)

  oleander
  ˌəulɪˈændə n. an evergreen poisonous shrub, Nerium oleander,
  native to the Mediterranean and bearing clusters of white, pink, or red
  flowers. [med.L]
  
  
  

From Webster's Revised Unabridged Dictionary (1913) (link)

  Oleander \O`le*an"der\, n. [F. ol['e]andre (cf. It. oleandro,
     LL. lorandrum), prob. corrupted, under the influence of
     laurus laurel, fr. L. rhododendron, Gr. ?; ? rose + ? tree.]
     (Bot.)
     A beautiful evergreen shrub of the Dogbane family, having
     clusters of fragrant red or white flowers. It is native of
     the East Indies, but the red variety has become common in the
     south of Europe. Called also rosebay, rose laurel, and
     South-sea rose.
  
     Note: Every part of the plant is dangerously poisonous, and
           death has occured from using its wood for skewers in
           cooking meat.

From Soule's Dictionary of English Synonyms (link)

  oleander
  n.
   Rose-bay (Nerium oleander).