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

ZnO + H₂O + 2OH^- --> [Zn(OH)₄]^2-

See also:

• amphiprotic
• ampholytic