The state that exists when acidic and basic ions in solution exactly neutralize each other is called acid -base equilibrium. 
Consider an acid, HA, in water. According to the Brønsted-Lowry acid/base theory, the acid should protonate the water to form hydronium and the conjugate base, A-. There will be an equilibrium between the acid and water, and hydronium and the conjugate base.
          This equilibrium can be used to calculate the concentrations of species in the solution.

When H+ ions and OH- ions are equal in nimber in a solution i.e. it is neutralised the it said to be in acid-base equilibrium.