Linseed is the small, reddish-brown or golden-yellow seed of the common flax plant, linum usitatissimum, the same plant that's used to grow the fibre from which linen has been made for thousands of years. Some flax plants produce more seeds than others, and these have carefully been chosen as a crop plant. It's equally correct to call the same small seeds linseed or flaxseed.
Traditionally pressed to make linseed oil, the seeds have long been used medicinally to help with constipation, but are packed with many other health benefits. They are more likely to be recommended as a vegetarian source of an important omega-3 fatty acid usually found in fish oils, but the accessibility of this ingredient can be affected by association with other constituents, and it’s best to take specialist dietary advice if this is important.
Linseeds add marked dietary fibre to your diet, and thus should first be added in modest amounts and always with increased liquid intake.