In chemistry, phosphorus oxoacid (or phosphorus acid) is a generic name for any acid whose molecule consists of atoms of phosphorus, oxygen, and hydrogen.  There is a potentially infinite number of such compounds.  Some of them are unstable and have not been isolated, but the derived anions and organic groups are present in stable salts and esters. The most important ones—in biology, geology, industry, and chemical research—are the phosphoric acids, whose esters and salts are the phosphates.
In general, any hydrogen atom bonded to an oxygen atom is acidic, meaning that the –OH group can lose a proton  leaving a negatively charged – group and thus turning the acid into a phosphorus oxoanion.  Each additional proton lost has an associated acid dissociation constant  K, K K, ..., often expressed by its cologarithm (pK, pK, pK, ...). Hydrogen atoms bonded directly to phosphorus are generally not acidic.