What is defined as the minimal point of soil moisture that a plant requires?

The permanent wilting point, also referred to as the wilting coefficient, represents the minimum amount of soil moisture a plant must have in its rooting zone in order to prevent an irreversible loss of turgor. At this critical point-aerobic matric potential around -15 bars-plants cannot take up water fast enough to survive, and they will not recover even when the air is fully saturated with moisture.