Certain hydrogen-containing molecules or ions react by transferring a hydrogen ion, H⁺, to another molecule or ion.
The reactant which is the source of the H⁺ is called an acid, the reactant to which the H⁺ becomes attached is called a base, and the reaction is called acid-base.

Example:

The acid hydrogen chloride tranfers H⁺ to the base ammonia to give the ionic solid ammonium chloride (composed of NH₄⁺ and Cl¯ ions).

The gas hydrogen chloride acts as an acid as it tranfers H⁺ to a water molecule which is acting as a base.

A hydrogen ion, H⁺, is just a proton, and these acid-base reactions are called proton transfer reactions. An acid is defined as a proton donor and a base is defined as a proton acceptor. This is the Brønsted-Lowry definition. A base must have a non-bonding pair of electrons which can be used to form a bond with a proton.

Brønsted acid: a proton donor, i.e. a species which donates a proton in a chemical reaction Example: HCl in the above examples.

Brønsted base: a proton acceptor, i.e a species which accepts a proton in a chemical reaction
Example: NH₃ and H₂O in the above two examples.

Oxonium (hydronium ion): the cation H₃O⁺, often written as H⁺(aq) or just H⁺ in equations in water (water, aqua, aq).
It is important to realise the proton, H⁺, does not exist as such in condensed matter, i.e. liquid or solid state. The term hydronium is still widely used

Hydroxide ion: the anion HO¯ (more commonly written OH¯, but it is O which has the formal charge of –1).

Conjugate acids and bases: When an acid loses a proton, its product is called the conjugate base of that acid. When a base accepts a proton its conjugate acid is formed
Example: HCl/Cl¯, NH₄⁺/NH₃, H₃O⁺/H₂O, and H₂O/OH¯ are conjugate acid-base pairs.

Neutralisation: The reaction between an acid and a base
Example: reaction of aqueous HCl and NaOH to give NaCl and H₂O .

Salt: the product (other than water) of a neutralisation reaction between an acid and a base. Salts are ionic compounds
Example: NaCl in the above neutralisation reaction.

Alkali: Aqueous solution of a base
Example: aqueous NaOH or aqueous ammonia .