In chemistry, it means a substance that can act as either acid or base. An example is amino acids and water. Similar many metals, such as zinc, tin, aluminium or beryllium. E.g. Zinc-oxide ZnO reacts differently depending on the pH of the solution:
In acids:ZnO + 2H+ --> Zn2+ + H2O
In bases:ZnO + H2O + 2OH- --> [Zn(OH)4]2-
This effect can be used to separate different cations, such as zinc from manganese.