Brønsted-Lowry concept of acids and bases, proposed in 1923 by Johannes Brønsted and Thomas Lowry, defines acids and bases based on their ability to donate or accept protons (H+)

Acid: A substance that donates a proton (H+)to another substance.

Base: A substance that accepts a proton (H+) from another substance.

When hydrochloric acid (HCl) dissolves in water:

HCl (acid) + H2O -----> H3O (positive) + Cl (Negative)

HCl donates a proton to H2O, making HCl the acid and H2O the base

H3O(positive) is the conjugate acid and Cl (negative) is conjugate base