pH is a measure of how acidic or basic (alkaline) a substance is. The pH scale ranges from 0 to 14, with 7 being neutral. A pH below 7 is acidic and a pH above 7 is basic. Examples of substances with different pH values include lemon juice (pH around 2), water (pH 7), and bleach (pH around 12).
Very Simple Definition:
A measure of how acidic or basic a substance is. pH values range from 0 to 14, with 7 being neutral, values below 7 are acidic, and values above 7 are basic (for example, lemon juice has a pH of about 2, while baking soda has a pH of about 9).