Source : Webster's Revised Unabridged Dictionary (1913)

Tantamount \Tan"ta*mount`\, a. [F. tant so much (L. tantus) + E.
   amount.]
   Equivalent in value, signification, or effect.

         A usage nearly tantamount to constitutional right.
                                                  --Hallam.

         The certainty that delay, under these circumstances,
         was tantamount to ruin.                  --De Quincey.

Tantamount \Tan"ta*mount`\, v. i.
   To be tantamount or equivalent; to amount. [Obs.] --Jer.
   Taylor.

Source : WordNet®

tantamount
     adj : being essentially equal to something; "it was as good as
           gold"; "a wish that was equivalent to a command"; "his
           statement was tantamount to an admission of guilt"
           [syn: {equivalent}, {tantamount(p)}]