The Collaborative International Dictionary of English v.0.48:

Natal plum \Na*tal" plum`\
   1. (Bot.) The drupaceous fruit of two South African shrubs of
      the genus Carissa (formerly Arduina) (Carissa
      bispinosa and Carissa grandiflora). It is also called
      amatungulu.
      [1913 Webster]

   2. the shrub bearing the natal plum[1]; a very large
      closely branched South African shrub (Carissa
      grandiflora) of the dogbane family having forked bright
      green spines, white flowers, shiny leaves, and red
      berries.
      [WordNet 1.5 +PJC]

The Collaborative International Dictionary of English v.0.48:

amatungulu \amatungulu\ n.
   1. a very large closely branched South African shrub
      (Carissa grandiflora) of the dogbane family having
      forked bright green spines, white flowers, shiny leaves,
      and red berries. Also called natal plum.

   Syn: natal plum, Carissa macrocarpa.
        [WordNet 1.5 +PJC]

   2. the fruit of the amatungulu[1].
      [PJC]

WordNet (r) 3.0 (2006):

natal plum
    n 1: very large closely branched South African shrub having
         forked bright green spines and shiny leaves [syn: natal
         plum, amatungulu, Carissa macrocarpa, Carissa
         grandiflora]
    2: South African shrub having forked spines and plumlike fruit;
       frequently used as hedging [syn: hedge thorn, natal plum,
       Carissa bispinosa]
    3: edible scarlet plumlike fruit of a South African plant [syn:
       carissa plum, natal plum]