The Collaborative International Dictionary of English v.0.48:

Plethora \Pleth"o*ra\, n. [NL., fr. Gr. ?, fr. ? to be or become
   full. Cf. Pleonasm.]
   1. Overfullness; especially, excessive fullness of the blood
      vessels; repletion; that state of the blood vessels or of
      the system when the blood exceeds a healthy standard in
      quantity; hyper[ae]mia; -- opposed to an[ae]mia.
      [1913 Webster]

   2. State of being overfull; excess; superabundance.
      [1913 Webster]

            He labors under a plethora of wit and imagination.
                                                  --Jeffrey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plethora
    n 1: extreme excess; "an embarrassment of riches" [syn:
         overplus, plethora, superfluity, embarrassment]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "plethora":
   amplitude, avalanche, congestion, deluge, embarras de richesses,
   engorgement, enough, extravagance, extravagancy, flood, flood tide,
   full, fullness, glut, high tide, high water, hyperemia, impletion,
   inundation, landslide, lavishness, many, money to burn,
   more than enough, much, overabundance, overaccumulation,
   overbounteousness, overbrimming, overburden, overcharge,
   overcopiousness, overdose, overflow, overfreight, overfullness,
   overkill, overlavishness, overload, overluxuriance, overmeasure,
   overmuch, overmuchness, overnumerousness, overplentifulness,
   overplenty, overplus, overpopulation, overprofusion, overspill,
   oversufficiency, oversupply, overweight, plenitude, plenty,
   prodigality, redundancy, repletion, satiety, saturation,
   saturation point, spate, spring tide, superabundance, superfluity,
   superflux, supersaturation, surcharge, surfeit, surplus,
   surplusage