Flax seed: The seed of the common flax plant (also known as linseed and, botanically, Linum usitatissimum). Flax seed and flax seed oil are a rich souce of ALA (alpha-linolenic acid), the only one of the omega-3 fatty acids that is essential in the sense that the body cannot make it so, if one is to get it, it must be in the diet.
