The reaction of an acid in water solvent is often described as a dissociation
where HA is a proton acid such as acetic acid, CHCOOH. The double arrow means that this is an equilibrium process, with dissociation and recombination occurring at the same time. This implies that the acid dissociation constant
However a more explicit description is provided by the Brønsted–Lowry acid–base theory, which specifies that the proton H+ does not exist as such in solution but is instead accepted by (bonded to) a water molecule to form the hydronium ion HO.
The reaction can therefore be written as
and better described as an ionization or formation of ions (for the case when HA has no net charge). The equilibrium constant is then
where  is not included because in dilute solution the solvent is essentially a pure liquid with a thermodynamic activity of one.
K is variously named a dissociation constant, an acid ionization constant, an acidity constant or an ionization constant. It serves as an indicator of the acid strength: stronger acids have a higher K value (and a lower pK value).