The Collaborative International Dictionary of English v.0.48:

Tantamount \Tan"ta*mount`\, a. [F. tant so much (L. tantus) + E.
   amount.]
   Equivalent in value, signification, or effect.
   [1913 Webster]

         A usage nearly tantamount to constitutional right.
                                                  --Hallam.
   [1913 Webster]

         The certainty that delay, under these circumstances,
         was tantamount to ruin.                  --De Quincey.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tantamount \Tan"ta*mount`\, v. i.
   To be tantamount or equivalent; to amount. [Obs.] --Jer.
   Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tantamount
    adj 1: 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)]