pH is a quantitative scale used to specify the acidity or basicity of an aqueous solution. It is defined as the negative logarithm of the hydrogen ion activity, providing a numerical measure of the concentration of $\text{H}^+$ ions. The scale typically ranges from 0 to 14, where a pH below 7 indicates an acidic solution due to a higher concentration of hydrogen ions, while a pH above 7 denotes a basic or alkaline solution. A pH of 7 is considered neutral. This measurement is fundamental across multiple scientific disciplines, including chemistry, geology, and biology, where it is essential for monitoring chemical equilibrium in natural systems and physiological processes.