any of the major appearances or aspects in which a thing of varying modes or conditions manifests itself to the eye or mind. 2. a stage in a process of change or development: Each phase of life brings its own joys. 3. a side, aspect, or point of view: This is only one phase of the question.
The pH of a neutral solution is 7. At this pH, the solution has equal concentrations of both the H+ and OH- ions, which is 10^-7 mole/litre. The presence of both the hydrogen and hydroxyl ions in equimolar concentrations makes the solution neutral.
That's why 7 is neutral. For those who want a more complicated answer, pH is defined: pH = -log10[H+], where [H+] is the concentration of H+ , expressed in moles/liter. In pure water near room temperature, the concentration of H+ is about 10-7 moles/liter, which gives a pH of 7.