buffer solution

noun

: a solution that usually contains on the one hand either a weak acid (as carbonic acid) together with one of the salts of this acid or with at least one acid salt of a weak acid or on the other hand a weak base (as ammonia) together with one of the salts of the base and that by its resistance to changes in hydrogen-ion concentration on the addition of acid or base is useful in many chemical, biological, and technical processes