There are 3 different ways to define an acid or a base. Each definition (from top to bottom) is more broad than the next, allowing for a greater range of substances to be classified as an acid or base. It is important to note that substances classified in in the narrow sense of an Arrhenius acid or base is also inclusive to the other definitions; however the reverse, Lewis acids and bases MAY not be considered Brønsted-Lowry and/or Arrhenius acids or bases. (See Venn diagram below for illustration of this point.)

Arrhenius

Acid = when dissolved in water, increases the concentration of hydrogen ions (H+)

Base = when dissolved in water, increases the concentration of hydroxide ions (OH-)

Brønsted-Lowry      *most commonly referenced definition

Acid = donates a proton (H+ ion) to another substance during a chemical reaction

Base = accepts a proton (H+ ion) from another substance during a chemical reaction

Lewis

Acid = accepts a pair of electrons

Base = donates a pair of electrons

The Lewis definition is less commonly used. This definition is often applicable to various solvents, not just aqueous solutions, and plays a crucial role in understanding coordination complexes