The Ionic Product of Water, Kw, is the equilibrium constant for the reaction in which water undergoes an acid-base reaction with itself. That is, water is behaving simultaneously as both an acid and a base.
H2O(l) + H2O(l) = H3O+(aq) + OH-(aq)
Kw = [H3O+(aq)][OH-(aq)]
At 298 K, the value of Kw is 1 x 10-14 mol2 dm-6. This makes the concentration of H+ ions equal to 1 x 10-7 mol dm-3, and therefore the pH is 7. This is defined as 'neutral'.
From the above equilibrium expression, taking -log10 throughout
pKw = pH + pOH = 14
Ionic Product does not apply only to water. It applies, for example, to the equilibrium in liquid ammonia:
NH3 + NH3 = NH2- + NH4+
See also pH.