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]