Orthophosphoric acid refers to phosphoric acid, which is the IUPAC name for this compound. The prefix ortho is used to distinguish the acid from related phosphoric acids, called polyphosphoric acids.
O 1339 (OTTO) Orthophosphoric acid, for HPLC, 85%+ Cas 7664-38-2 - used as a flux by hobbyists (such as model railroaders) as an aid to soldering.