Dictionary Definition:

“An acid is a substance that donates protons and a base is a substance that accepts protons.”

Selected from OxfordDictionaries.com

Functional Definition:

Brønsted-Lowry acids and bases were first introduced in general chemistry. This concept of acids and bases involves the shifting of protons (H) between two molecules.  Brønsted-Lowry acids typically donate a proton such as H-Cl (a strong acid)or CH3-COOH (a weak acid). Brønsted-Lowry bases are proton exceptors like Na-OH (strong base) or NH3 (weak base).