equivalent, as in value, force, effect, or signification: His angry speech was tantamount to a declaration of war.

1635–45; adj. use of obsolete noun: that which amounts to as much, itself noun use of obsolete v.: to amount to as much < Anglo-French tant amunter or Italian tanto montare to amount to as much. See tanto, amount

paramount, tantamount, ultimately.

See equal.
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World English Dictionary
tantamount (ˈtæntəˌmaʊnt)
adj (foll by to)
as good (as); equivalent in effect (to): his statement was tantamount to an admission of guilt
[C17: basically from Anglo-French tant amunter to amount to as much, from tant so much + amunter to amount]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1641, from verbal phrase tant amount "be equivalent" (1628), from Anglo-Fr. tant amunter "amount to as much" (1292), from O.Fr. tant "as much" (from L. tantus, from tam "so") + amonter "amount to, go up" (see amount).
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
As a result, captive panda studies are of tantamount importance.
That's because a fully functioning ecology is tantamount to tangible wealth.
They trust each other with their careers, which among many academics is
  tantamount to trusting each other with their lives.
Inquiring into and articulating the factors that contribute to crime isn't
  tantamount to excusing it.
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