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