n 1  :  a syrupy or deliquescent tribasic acid H₃PO₄ used esp. in preparing phosphates (as for fertilizers), in rust-proofing metals, and esp. formerly as a flavoring in soft drinks - called also orthophosphoric acid   2  :  a compound (as pyrophosphoric acid or metaphosphoric acid) consisting of phosphate groups linked directly to each other by oxygen .
 
 
 
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