The Collaborative International Dictionary of English v.0.48:

Soda water \So"da wat`er\, n.
   1. originally, a beverage consisting of a weak solution of
      sodium bicarbonate to which an acid (such as phosphoric
      acid) has been added to generate carbon dioxide and thus
      cause effervescence; the term now is also used as a
      synonym for soda pop. Soda water without flavoring is
      used as a beverage or as a component of a mixed beverage.
      It is typically bottled or canned under pressure so that
      the carbon dioxide and effervescence will be preserved
      until use.
      [PJC]

   2. a beverage containing supersaturated carbon dioxide
      (carbonic acid) and thus effervescent.
      [PJC]

   3. same as soda pop.
      [PJC]

WordNet (r) 3.0 (2006):

soda water
    n 1: effervescent beverage artificially charged with carbon
         dioxide [syn: soda water, carbonated water, club
         soda, seltzer, sparkling water]
    2: a sweet drink containing carbonated water and flavoring; "in
       New England they call sodas tonics" [syn: pop, soda,
       soda pop, soda water, tonic]