Wiktionary
polyphosphoric acid
n. (context chemistry English) Any of several ill-defined polymers of orthophosphoric acid; they are viscous, hygroscopic liquids
WordNet
polyphosphoric acid
n. a series of oxyacids of phosphorus
Usage examples of "polyphosphoric acid".
To get an idea of what's happening, have a chemist drop polyphosphoric acid in water.