Wordnet 3.0
NOUN (2)
1. 
 a journey by ox wagon (especially an organized migration by a group of settlers); 
2. 
 any long and difficult trip; 
VERB (2)
1. 
 journey on foot, especially in the mountains; 
- Example: "We spent the summer trekking in the foothills of the Himalayas"2. 
 make a long and difficult journey; 
- Example: "They trekked towards the North Pole with sleds and skis"
The Collaborative International Dictionary of English v.0.48:
Trek \Trek\, v. i. [imp. & p. p. Trekked; p. pr. & vb. n.
   Trekking.] [Written also treck.] [D. trekken. See
   Track, n.] [South Africa]
   1. To draw or haul a load, as oxen.
      [Webster 1913 Suppl.]
   2. To travel, esp. by ox wagon; to go from place to place; to
      migrate. [Chiefly South Africa]
            One of the motives which induced the Boers of 1836
            to trek out of the Colony.            --James Bryce.
      [Webster 1913 Suppl.]
The Collaborative International Dictionary of English v.0.48:
Trek \Trek\, n. [Written also treck.] [D. Cf. Track, n.]
   The act of trekking; a drawing or a traveling; a journey; a
   migration. [Chiefly South Africa]
         To the north a trek was projected, and some years later
         was nearly carried out, for the occupation of the
         Mashonaland.                             --James Bryce.
   Great Trek, the great emigration of Boers from Cape Colony
      which began in 1836, and resulted in the founding of the
      South African Republic and Orange Free State.
      [Webster 1913 Suppl.]
WordNet (r) 3.0 (2006):
trek
    n 1: a journey by ox wagon (especially an organized migration by
         a group of settlers)
    2: any long and difficult trip
    v 1: journey on foot, especially in the mountains; "We spent the
         summer trekking in the foothills of the Himalayas"
    2: make a long and difficult journey; "They trekked towards the
       North Pole with sleds and skis"
Moby Thesaurus II by Grady Ward, 1.0:
70 Moby Thesaurus words for "trek":
   campaign, circuit, course, cruise, emigrate, emigration, excursion,
   expatriate, expatriation, expedition, fare, flight, flit,
   globe-trot, go abroad, go on safari, go overseas, grand tour,
   hit the trail, immigrate, immigration, in-migrate, in-migration,
   intermigrate, intermigration, jaunt, journey, junket,
   make a journey, make a pilgrimage, make a trip, migrate, migration,
   out-migrate, out-migration, outing, package tour, passage,
   peregrinate, peregrination, peregrinations, pilgrim, pilgrimage,
   pleasure trip, progress, range the world, remigrate, remigration,
   round trip, rubberneck, rubberneck tour, run, safari, sally, shoot,
   sight-see, stalk, swarm, swarming, take a trip, take the road,
   take wing, tour, transmigrate, transmigration, travels, trip, turn,
   voyage, wayfare