Najis (نجس) means ritually impure; the impurity itself is called najasah. Examples include pork, blood, urine and, according to many scholars, alcoholic drinks. Impure substances affect the validity of prayer if they are on the body, clothes or place of prayer.
This is different from haram: something can be impure without being food, and scholars differ on whether ethanol in perfume or sanitiser is impure.