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)]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "tantamount":
   alike, all one, all the same, analogous, answering, coequal,
   complemental, complementary, convertible, coordinate,
   correspondent, corresponding, duplicate, equal, equiparant,
   equipollent, equivalent, homologous, identic, identical,
   indistinguishable, like, much the same, reciprocal, reciprocative,
   selfsame, uniform, very