Definition of "flax"

A plant of the genus Linum, especially Linum usitatissimum, which has a single, slender stalk, about a foot and a half high, with blue flowers. Also known as linseed, especially when referring to the seeds.  examples

The fibers of Linum usitatissimum, grown to make linen and related textiles.  examples

The flax bush, a plant of the genus Phormium, native to New Zealand, with strap-like leaves up to 3 metres long that grow in clumps.  examples