A polyatomic anion
Meaning it is a negatively charged molecule consisting of two or more elements, one of which is oxygen.
Examples: NO2- Nitrite ion, ClO3- Chlorite ion
When ionized in water these ions create an acidic solution. The acidity of the solution they will create can be predicted by looking at the oxidation number of the central atom. (The central atom will be the one that is not oxygen or hydrogen.) This applies only for the oxoanions of non-metals.
The bigger the oxidation number of the central atom, the more acidic the solution will be.
Another way to predict the acidity of the solution is to look at the electronegativity of the central atom.
The more electronegative the central atom, the more acidic the solution will be.