Flaxseed, also called linseed, edible seeds harvested from flax (Linum usitatissimum) plants, used as a health food and as a source of linseed, or flaxseed, oil. Flax plants grown for their seeds are generally shorter, have more branches, and produce more seeds than those that are grown primarily for linen fibre. They are usually brown and are smooth and shiny, with a mucilaginous substance in their outer layer that makes them sticky when wet.