$HBr(aq)$ is an acid according to the Arrhenius definition.
Work Step by Step
According to the Arrhenius definition, an acid is a substance which produces $H^+$ ions in an aqueous solution. As we can see from the below equation, $HBr(aq) \rightarrow H^+(aq) + Br^-(aq)$ $HBr$ produces $H^+$ ions in aqueous solutions. So it is an acid.