The Collaborative International Dictionary of English v.0.48:

Deceit \De*ceit"\, n. [OF. deceit, des[,c]ait, decept (cf.
   deceite, de[,c]oite), fr. L. deceptus deception, fr.
   decipere. See Deceive.]
   1. An attempt or disposition to deceive or lead into error;
      any declaration, artifice, or practice, which misleads
      another, or causes him to believe what is false; a
      contrivance to entrap; deception; a wily device; fraud.
      [1913 Webster]

            Making the ephah small and the shekel great, and
            falsifying the balances by deceit.    --Amos viii.
                                                  5.
      [1913 Webster]

            Friendly to man, far from deceit or guile. --Milton.
      [1913 Webster]

            Yet still we hug the dear deceit.     --N. Cotton.
      [1913 Webster]

   2. (Law) Any trick, collusion, contrivance, false
      representation, or underhand practice, used to defraud
      another. When injury is thereby effected, an action of
      deceit, as it called, lies for compensation.

   Syn: Deception; fraud; imposition; duplicity; trickery;
        guile; falsifying; double-dealing; stratagem. See
        Deception.
        [1913 Webster]

WordNet (r) 3.0 (2006):

deceit
    n 1: the quality of being fraudulent [syn: fraudulence,
         deceit]
    2: a misleading falsehood [syn: misrepresentation, deceit,
       deception]
    3: the act of deceiving [syn: deception, deceit,
       dissembling, dissimulation]