In general, this term describes a process that does not proceed in the same way when run in different directions; a simple example would be that you do not get the same amount of energy out of an elastic band when you allow it to relax as when you stretch it (some energy goes to heat the elastic band). Hysteresis is often used in wind energy to describe the way in which, whilst a WT may cut out when the wind speed reaches 25m/s, it will not cut back in again until the wind speed drops below (for example) 23m/s. There is also a hysteresis loop involved in blade behaviour in dynamic situations (see Dynamic Stall).