The term pH comes for Hydrogen potential, and is written pH. It defines whether a water-based product is acidic or basic.
Pure water at 25°C (77°F) is the reference of a pH equal to 7: this figure is the neutral pH.
A figure lower than 7 corresponds to an acidic product, a figure higher than 7 is a basic product.
A skin in good health has a slightly acidic figure, 6 to 6.5 maximum. This is a way for the hydrolipidic film to protect naturally the skin against the development of bacteria, which we are constantly in contact with.
This means of defence comes slowly: a baby’s skin pH is close to 7. The balance is gotten at puberty, the figure being ca 5.5. A more alkaline pH may lead to problems or skin diseases.