An older definition of acids as substances which increase the concentration of hydrogen ions when dissolved in water, and bases as substances which increase the concentration of hydroxide ions.
Proton Donor: A substance that donates a proton or H+ ion to another substance; according to Brønsted-Lowry theory, this is an acid.
Proton Acceptor: A substance that accepts a proton or H+ ion from another substance; according to Brønsted-Lowry theory, this is a base.