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A phosphoric ester is an ester of phosphoric acid: an organic group is joined through oxygen to phosphorus. Replacing one, two or three of the acid's O–H hydrogens by organic groups gives a phosphate monoester, phosphate diester or phosphate triester. A phosphoric anhydride instead contains an acid-derived linkage, such as P–O–P, rather than an additional P–O–C ester linkage.^[ref]

For a structural counting example, compareThe first has one organic O–P attachment; the second has two. Removing an O–H proton changes the charge but leaves that attachment count unchanged. Conversely, adding a second separate monoester group elsewhere on a large molecule does not turn the first phosphorus center into a diester.

This distinction matters in biochemical measurements. 4-Nitrophenyl phosphate is a phosphate monoester whose cleavage can be followed optically; the released inorganic phosphate is a different chemical species. An alkaline phosphatase assay detects a specified conversion under specified conditions, rather than counting all phosphorus-containing material indiscriminately.^[ref]