Acidity (noun)

1. A sour taste.
2. The quality of being sour, sharp, or tangy, especially in taste.
3. A measure of the concentration of hydrogen ions in a solution, indicating its acidity or alkalinity.

Origin:

Late middle english (in the sense "acridity, sourness"): from old french acidite or latin aciditas, from acidus (see acid).

Examples:

1. The acidity of the vinegar is important in the recipe.
2. The acidity in his voice made it clear that he was angry.
3. Too much acidity in the soil can be harmful to plants.
4. The wine had a good balance of fruit and acidity.
5. Acidity levels in the human body must be kept in check for good health.