Answer: a substance that increases the hydroxide ion concentration of a solution
Explanation: According to Arrhenius concept, an acid is a substance which donates hydronium ions ions when dissolved in water and base is a substance which donates hydroxide ions
ions in water.
According to the Bronsted Lowry conjugate acid-base pair, an acid is a substance that donates protons and a base is a substance that accepts protons.
According to Lewis concept, an acid is the substance which accepts electron pairs and abase is a substance which donates electron pairs.
Thus a substance that increases the hydroxide ion concentration of a solution is an Arrhenius base.