The Collaborative International Dictionary of English v.0.48:

Vegetation \Veg`e*ta"tion\, n. [Cf. F. v['e]g['e]tation, L.
   vegetatio an enlivening. See Vegetable.]
   [1913 Webster]
   1. The act or process of vegetating, or growing as a plant
      does; vegetable growth.
      [1913 Webster]

   2. The sum of vegetable life; vegetables or plants in
      general; as, luxuriant vegetation.
      [1913 Webster]

   3. (Med.) An exuberant morbid outgrowth upon any part,
      especially upon the valves of the heart.
      [1913 Webster]

   Vegetation of salts (Old Chem.), a crystalline growth of an
      arborescent form.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vegetation
    n 1: all the plant life in a particular region or period;
         "Pleistocene vegetation"; "the flora of southern
         California"; "the botany of China" [syn: vegetation,
         flora, botany] [ant: fauna, zoology]
    2: the process of growth in plants
    3: an abnormal growth or excrescence (especially a warty
       excrescence on the valves of the heart)
    4: inactivity that is passive and monotonous, comparable to the
       inactivity of plant life; "their holiday was spent in sleep
       and vegetation"

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "vegetation":
   a wise passiveness, abeyance, apathy, botany, budding, burgeoning,
   catalepsy, catatonia, contemplation, contemplative life,
   deadliness, deathliness, development, do-nothing policy,
   do-nothingism, do-nothingness, dormancy, entropy, excrescence,
   flora, flowerage, gemmation, germination, greenery, greens, growth,
   herbage, idleness, immobility, inaction, inactivity, indifference,
   indolence, inertia, inertness, just being, laissez-aller,
   laissez-faire, laissez-faireism, languor, latency, lotus-eating,
   luxuriation, maturation, meditation, mere existence, mere tropism,
   neutralism, neutrality, neutralness, noninvolvement,
   nonparticipation, nonresistance, nonviolence,
   nonviolent resistance, outgrowth, overgrowth, pacifism, paralysis,
   passive resistance, passive self-annihilation, passiveness,
   passivism, passivity, plant kingdom, plant life, plants, policy,
   procrastination, procreation, pullulation, quiescence, quietism,
   reproduction, sloth, sprouting, stagnancy, stagnation,
   standpattism, stasis, suspense, torpor, upgrowth,
   vegetable kingdom, vegetable life, vegetation spirit, verdure,
   vis inertiae, vita contemplativa, waiting game,
   watching and waiting