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]

WordNet (r) 3.0 (2006):

Carissa bispinosa
    n 1: South African shrub having forked spines and plumlike
         fruit; frequently used as hedging [syn: hedge thorn,
         natal plum, Carissa bispinosa]