Amphiprotic Species
A species that can both donate and accept a proton.
Unit
Dimensionless
In depth
Amphiprotic behaviour is a specific type of amphoterism restricted to species that actually transfer protons, so it requires both an ionisable hydrogen and a lone pair. Whether such a species behaves as acid or base in a given solution depends on the relative sizes of its Ka and Kb.
Examples in the real world
HCO₃⁻, H₂PO₄⁻ and water itself; hydrogencarbonate is basic in water because its Kb exceeds its Ka.