A species that has the potential to act both as an acid and as a base according to Brønsted-Lowry Theory is said to be amphoteric.

eg:     Water

The water molecule has hydrogen atoms and, therefore, could act as an acid in a reaction.  The oxygen atom in the water molecule has two lone pairs, one of which could be used to form a bond with a H⁺, and, therefore, the water molecule could act as a base in a reaction.  Since water has the potential to act both as an acid and as a base, water is amphoteric.