The Collaborative International Dictionary of English v.0.48:

Trekker \Trek"ker\, n. [D.]
   One that treks. [Written also trecker.] [South Africa]
   --James Bryce.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

trekker
    n 1: a traveler who makes a long arduous journey (as hiking
         through mountainous country)