Any factors of production whose quantities cannot be changed during the time horizon being analysed. In the short run the number of fixed factors will be larger than in the medium run. For example, the quantity of labour and intensity of capital use might be variable in the short run, the quantity of capital equipment variable in the medium run, but the size of the factory to house the capital and labour only variable in the long run. The long run can be defined as the time horizon over which all factors of production are variable.
Reference: Oxford Press Dictonary of Economics, 5th edt.