Learner's definition of FLAX. [noncount]. 1. : a plant that has blue flowers and that is grown for its fiber and its seed ◊ The fiber of flax is used to make linen and the seed is used to make linseed oil.◊ The fiber of flax is used to make linen and the seed is used to make linseed oil.2. : the fiber of the flax plant. a plant that has blue flowers and that is grown for its fiber and its seed ◊ The fiber of flax is used to make linen and the seed is used to make linseed oil. ◊ The fiber of flax is used to make linen and the seed is used to make linseed oil. 2. : the fiber of the flax plant.