More specifically, we use the term pH to express the acidity of a solution. pH is a measure of the hydrogen ion concentration, and it's calculated using the following formula:

pH = -log[H+]

where [H+] is the concentration of hydrogen ions in moles per liter (M).

Here's a breakdown:

* High [H+] means low pH, indicating a strong acid.

* Low [H+] means high pH, indicating a strong base.

So, while acidity refers to the general concept of hydrogen ion concentration, pH is the specific measurement used to quantify it.