Flax, also known as common flax or linseed, is a member of the genus Linum in the family Linaceae. It is a food and fiber crop cultivated in cooler regions of the world. The textiles made from flax are known in the Western countries as linen, and traditionally used for bed sheets, underclothes, and table linen. The oil is known as linseed oil. In addition to referring to the plant itself, the word "flax" may refer to the unspun fibers of the flax plant. The plant species is known only as a cultivated plant, and appears to have been domesticated just once from the wild species Linum bienne, called pale flax.

Cultivated flax plants grow to 1.2 m (3 ft 11 in) tall, with slender stems. The leaves are glaucous green, slender lanceolate, 20–40 mm long, and 3 mm broad.

The flowers are pure pale blue, 15–25 mm in diameter, with five petals. The fruit is a round, dry capsule 5–9 mm in diameter, containing several glossy brown seeds shaped like an apple pip, 4–7 mm long.

Flax is grown for its seeds, which can be ground into a meal or turned into linseed oil, a product used as a nutritional supplement and as an ingredient in many wood-finishing products. Flax is also grown as an ornamental plant in gardens. Moreover, flax fibers are used to make linen.

Linseed meal, the byproduct of producing linseed oil from flax seeds, is used to feed livestock. It is a protein-rich feed for ruminants, rabbits, and fish.

A 100-gram portion of ground flaxseed supplies about 534 calories (2,230 kJ), 41 g of fat, 28 g of fiber, and 20 g of protein.

Flaxseed sprouts are edible and have a slightly spicy flavor profile. Excessive consumption of flaxseeds with inadequate amounts of water may cause bowel obstruction.

Three natural phenolic glucosides—secoisolariciresinol diglucoside, p-coumaric acid glucoside, and ferulic acid glucoside—can be found in commercial breads containing flaxseed.

One study of research published between 1990 and 2008 showed that consuming flaxseed or its derivatives may reduce total and LDL-cholesterol in the blood, with greater benefits in women and those with high cholesterol.