Linoleic Acid is a fatty acid that your body can't manufacture but needs for good health. If linoleic acid is excluded from the diet, retarded growth and skin lesions have been seen to occur in animals. Because of this need for this fatty acid, but the body's inability to manufacture it, it is termed an "essential fatty acid".
Source:
Barasi, M. (1997) Human Nutrition Oxford University Press P. 79