From Coastal Wiki
Definition of Alkalinity:
Capacity of natural water to neutralize acid (proton-accepting capacity).
This is the common definition for Alkalinity, other definitions can be discussed in the article
Alkalinity is measured by titration of a water sample with a dilute acid to a specific pH endpoint. Most often, alkalinity is a function of carbonate (CO32–), hydrogencarbonate (HCO3–), and hydroxide (OH–) concentrations, i.e., the carbonate-hydrogencarbonate buffering of the water. However, dissolved organic compounds, borates, phosphates, and silicates can also contribute to alkalinity.