Flax (also known as Common Flax or Linseed) is a member of the genus Linum in the family Linaceae. The New Zealand flax is unrelated. Flax originated in India and was first domesticated in the Fertile Crescent.

It is an erect annual plant growing to 120 cm tall, with slender stems. The leaves are glaucous green, slender lanceolate, 2-4 cm long and 3 mm broad. The flowers are pure pale blue, 1.5-2.5 cm diameter, with five petals. The fruit is a round, dry capsule 5-9 mm diameter, containing several glossy brown seeds shaped like an apple pip, 4-7 mm long.

In addition to the plant itself, flax may refer to the unspun fibres of the flax plant.

Flax is grown both for seed and for its fibers. Various parts of the plant have been used to make fabric, dye, paper, medicines, fishing nets and soap. It is also grown as an ornamental plant in gardens, as flax is one of the few plant species that is capable of producing truly blue flowers (most "blue" flowers are really shades of purple), although not all flax varieties produce blue flowers.

The seeds produce a vegetable oil known as linseed oil or flaxseed oil. It is one of the oldest commercial oils and solvent-processed flax seed oil has been used for centuries as a drying oil in painting and varnishing. Flax seeds come in two basic varieties; brown and yellow (also referred to as golden). Although brown flax can be consumed and has been for thousands of years, it is better known as an ingredient in paints, fibre and cattle feed. Brown and yellow flax have similar nutritional values and equal amounts of short-chain omega-3 fatty acids. The exception is a type of yellow flax called solin which is very low in omega-3 and has a completely different oil profile. A number of studies have shown that people have a very hard time absorbing the Omega-3 from flaxseed oil compared to oily fish (see Fish and plants as a source of Omega-3 for more).

A North Dakota State University research project led to the creation of a new variety of the yellow flax seed called "Omega". This new variety was created primarily as a food source and has a more pleasant nutty-buttery flavor than the brown variety and retains a comparable level of the beneficial Omega-3 oil.

One tablespoon of ground flax seeds and three tablespoons of water may serve as a replacement for one egg in baking by binding the other ingredients together, and ground flax seeds can also be mixed in with oatmeal, yogurt, water (similar to Metamucil), or any other food item where a nutty flavor is appropriate Flaxseed oil is most commonly consumed with salads or in capsules. Flax seed owes its nutritional benefits to
Lignans  and omega-3 essential fatty acids. Omega-3s, often in short supply in populations with low-fish diets, promote heart health by reducing cholesterol, blood pressure and plaque formation in arteries. In addition, flaxseed oil is often recommended as a galactagogue. Lignans benefit the heart and possess anti-cancer properties: A series of research studies by Lilian U. Thompson and her colleagues at the Department of Nutritional Science of the University of Toronto have reported that flaxseed can have a beneficial effect in reducing tumor growth in mice, particularly the kind of tumor found in human post-menopausal breast cancer. The effects are thought to be due to the lignans in flaxseed, and are additive with those of tamoxifen, the currently standard drug treatment for these cancers. Initial studies suggest that flaxseed taken in the diet have similar beneficial effects in human cancer patients. Reports that flaxseed is beneficial in other cancers, e.g. prostate cancer, are less numerous but promising.

Flax seed sprouts are edible, with a slightly spicy flavor.

Flax fibres are amongst the oldest fibre crops in the world. The use of flax for the production of linen goes back 5000 years. Pictures on tombs and temple walls at Thebes depict flowering flax plants. The use of flax fibre in the manufacturing of cloth in northern Europe dates back to pre-Roman times. In North America, flax was introduced by the Pilgrim fathers. Currently most flax produced in the USA and Canada are seed flax types for the production of linseed oil or flaxseeds for human nutrition.

Flax fibre is extracted from the bast or skin of the stem of flax plant. Flax fibre is soft, lustrous and flexible. It is stronger than cotton fibre but less elastic. The best grades are used for linen fabrics such as damasks, lace and sheeting. Coarser grades are used for the manufacturing of twine and rope. Flax fibre is also a raw material for the high-quality paper industry for the use of printed banknotes and rolling paper for cigarettes.

The major fibre flax-producing countries are the former USSR, Poland, France, Belgium, Ireland, and the Czech Republic.

The soils most suitable for flax, besides the alluvial kind, are deep friable loams, and containing a large proportion of organic matter. Heavy clays are unsuitable, as are soils of a gravelly or dry sandy nature.