What is permanent wilting percentage

As plants absorb water from a soil, they lose most of it through evaporation at the leaf surface.  Some water is also lost by evaporation directly from the soil surface.  As the soil dries, plants begin to wilt to conserve moisture during the day time. Initially, the plants regain their vigour at night, but ultimately they will remain wilted night and day.  Although not dead, the plants are in a permanent wilted condition and will die if water is not provided.  The moisture content of the soil at this stage is called  wilting coefficient or permanent wilting percentage.