Acidity

The bright, tart quality of a coffee, carried mostly by malic, citric and phosphoric acids. A quality in specialty coffee, a fault only when it is sour.

Acidity is not the same as sourness. Sourness is what an under extracted coffee does; acidity is a structured, fruit-like brightness that a washed Kenyan or Ethiopian lot carries deliberately. Altitude raises it, dark roasting destroys it.