An acid is any substance that donates protons or hydrogen ions to an aqueous solution, thereby increasing the hydronium ion concentration.
A substance that accepts protons or hydrogen ions from an aqueous solution, thereby decreasing the hydronium ion concentration.
A logarithmic scale used to specify the acidity or basicity/alkalinity of an aqueous solution. It measures how much H3O+ is present.
Buffer: A solution that resists changes in pH when acid or alkali is added to it; buffers are typically made up of a weak acid and its conjugate base.