The Collaborative International Dictionary of English v.0.48:

Landslide \Land"slide`\, Landslip \Land"slip`\, n.
   1. The slipping down of a mass of land from a mountain, hill,
      etc.
      [1913 Webster]

   2. The land which slips down.
      [1913 Webster]

   3. An election victory in which the winning candidate
      receives a substantial majority of the votes, usually
      meaning at least ten per cent more than any opposing
      candidate.
      [PJC]

   4. Any overwhelming victory.
      [PJC]

WordNet (r) 3.0 (2006):

landslide
    n 1: an overwhelming electoral victory; "Roosevelt defeated
         Hoover in a landslide"
    2: a slide of a large mass of dirt and rock down a mountain or
       cliff [syn: landslide, landslip]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "landslide":
   Cadmean victory, KO, Pyrrhic victory, abundance, affluence,
   ample sufficiency, ampleness, amplitude, ascendancy, avalanche,
   bonanza, bountifulness, bountiousness, bumper crop, championship,
   coast, conquest, copiousness, count, deluge, easy victory,
   election returns, embarras de richesses, enough, extravagance,
   extravagancy, exuberance, fertility, flood, flow, foison,
   full measure, fullness, generosity, generousness, glide, glissade,
   glissando, grand slam, great abundance, great plenty, gush,
   inundation, knockout, landslide victory, landslip, lavishness,
   liberality, liberalness, lots, luxuriance, mastery, maximum,
   money to burn, moral victory, more than enough, much, myriad,
   myriads, numerousness, official count, opulence, opulency,
   outpouring, overabundance, overaccumulation, overbounteousness,
   overcopiousness, overdose, overflow, overlavishness,
   overluxuriance, overmeasure, overmuchness, overnumerousness,
   overplentifulness, overplenty, overpopulation, overprofusion,
   oversufficiency, oversupply, picnic, plenitude, plenteousness,
   plentifulness, plenty, plethora, poll, prevalence, prodigality,
   productiveness, profuseness, profusion, pushover, quantities,
   recount, redundancy, repleteness, repletion, returns, rich harvest,
   rich vein, richness, riot, riotousness, runaway victory, scads,
   shower, sideslip, skid, slide, slip, slippage, slither, snowslide,
   snowslip, spate, stream, subdual, subduing, subsidence,
   substantiality, substantialness, success, superabundance,
   superflux, teemingness, tidal wave, total victory, triumph,
   victory, walkaway, walkover, wealth, win, winning,
   winning streak