There are three principle definitions for acids and bases.

The Arrhenius definition is the simplest, and states that acids are compounds that increase proton concentration in solution, while bases are compounds that increase hydroxide concentration in solution. Arrhenius acids will always contain hydrogen and Arrhenius bases will always contain hydroxide groups.

Brønsted-Lowry definitions center on proton donation. Brønsted-Lowry acids are species that donate protons, and Brønsted-Lowry bases are compounds that accept protons.

Lewis acids and bases refer to the donation or acceptance of an electron pair. Lewis bases donate an electron pair, and Lewis acids accept an electron pair.

An acidic solution will always have a hydronium concentration greater than . A basic solution will always have a hydroxide concentration greater than . If the hydroxide concentration is equal to the hydronium concentration, the solution is neutral.

The opposite of the above conditions can also be noted. If the hydronium concentration is less than , then the solution will be basic. An example of this is a solution where the hydronium ion concentration is . Since this value is far less than , this will be a basic solution.

Strong acids and bases dissociate completely into ions when placed in an aqueous solution. In contrast, weak acids and bases do not completely dissociate.

There are only six strong acids: perchloric acid (HClO₄), hydroiodic acid (HI), hydrobromic acid (HBr), hydrochloric acid (HCl), nitric acid (HNO₃), and sulfuric acid (H₂SO₄).

There are three primary classifications of acids and bases.

Arrhenius acids yield protons when dissolved in solution, while Arrhenius bases yield hydroxide ions.

Brønsted-Lowry acids are protone donors, while Brønsted-Lowry bases are proton acceptors.

Lewis acids are electron acceptors, while Lewis bases are electron donors.

An acid that dissociates completely in solution is considered a strong acid due to its high Ka value.

The most common Lewis bases are anions, and therefore, have unpaired valence electrons which may be donated to Lewis acids. The concept of donating and/or accepting hydrogens refers to the Bronsted-Lowry definition of acids. A Bronsted-Lowry acid donates hydrogens.

Acids are soluble in water substance, but remember, only water gets the liquid (l) phase label. Since acids are almost always dissolved in water, we use the (aq) subscript at the end of the chemical formula to indicate its phase.

According to the Lewis definition, acids are electron pair acceptors.

According to the Lewis definition, bases are electron pair donors. 

An amphoteric substance is one that can act either as an acid or a base. Amino acids are good examples of amphoteric substances; they have a carboxylic acid group that can be acidic, and an amino group that can be basic.

Acids have hydrogen ions and balanced charges, "di" tells us that the acid in question has two potential hydrogen ions, which can be ionized and turned into protons (protic). These acids, like  can ionize once, yielding  then again, yielding .