Here's a breakdown:

* pH is a measure of the hydrogen ion concentration in a solution.

* "p" indicates the negative logarithm of the hydrogen ion concentration.

* In other words, pH is the negative base-10 logarithm of the hydrogen ion activity.

Here's the mathematical expression:

pH = -log₁₀[H⁺]

where:

* [H⁺] is the hydrogen ion concentration in moles per liter (mol/L)

Why use a negative logarithm?

Using a negative logarithm allows us to express very small hydrogen ion concentrations in a more convenient way. Instead of dealing with extremely small numbers like 10⁻⁷, we can use a positive number like 7 to represent the same concentration.

In summary: The "p" in pH signifies that it's a logarithmic scale used to express the acidity or alkalinity of a solution based on its hydrogen ion concentration.