Here's why:

* Brønsted-Lowry Theory: NH3 can accept a proton (H+) to form the ammonium ion (NH4+). Bases are proton acceptors.

* Lewis Theory: NH3 has a lone pair of electrons on the nitrogen atom, making it capable of donating electrons to form a coordinate covalent bond. Lewis bases are electron donors.

In water, ammonia acts as a weak base:

NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH- (aq)

The presence of OH- ions in solution indicates that ammonia has acted as a base.