Concept:
According to the Bronsted-Lowry theory, an acid is a proton (\(H^+\)) donor, and a base is a proton acceptor.
Step 1: Analyze the species transformation.
The chemical equation is:
\[
NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-
\]
\(NH_3\) accepts a proton to become \(NH_4^+\), acting as a Bronsted-Lowry base.
\(H_2O\) donates a proton to become \(OH^-\), acting as a Bronsted-Lowry acid.
Step 2: Identify the role of water.
Since water acts as a proton donor in this specific chemical reaction:
Water behaves as a Bronsted-Lowry acid.
\[
\boxed{\text{A Bronsted-Lowry acid}}
\]