Under Lavoisiers original theory, all acids contained oxygen, which was named from the Greek ὀξύς (oxys: acid, sharp) and the root -γενής (-genes': creator). It was later discovered that some acids, notably hydrochloric acid, did not contain oxygen and so acids were divided into oxo-acids and these new hydroacids.
All oxyacids have the acidic hydrogen bound to an oxygen atom, so bond strength (length) is not a factor, as it is with binary nonmetal hydrides. Rather, the electronegativity of the central atom and the number of oxygen atoms determine oxyacid acidity. For oxyacids with the same central atom, acid strength increases with the number of oxygen atoms attached to it. With the same number of oxygen atoms attached to it, acid strength increases with increasing electronegativity of the central atom.
Compared to the salts of their deprotonated forms (a class of compounds known as the oxyanions), oxyacids are generally less stable, and many of them only exist formally as hypothetical species, or only exist in solution and cannot be isolated in pure form.  There are several general reasons for this: (1) they may condense to form oligomers (e.g., HCrO to HCrO), or dehydrate all the way to form the anhydride (e.g., HCO to CO), (2) they may disproportionate to one compound of higher and another of lower oxidation state (e.g., HClO to HClO and HClO), or (3) they might exist almost entirely as another, more stable tautomeric form (e.g., phosphorous acid P(OH) exists almost entirely as phosphonic acid HP(=O)(OH)).  Nevertheless, perchloric acid (HClO), sulfuric acid (HSO), and nitric acid (HNO) are a few common oxyacids that are relatively easily prepared as pure substances.
Imidic acids are created by replacing =O with =NR in an oxyacid.