Flax, also known as common flax or linseed, is a flowering plant, Linum usitatissimum, in the family Linaceae. It is cultivated as a food and fiber crop in regions of the world with temperate climates. In 2022, France produced 75% of the world's supply of flax.

Textiles made from flax are known in English as linen and are traditionally used for bed sheets, underclothes, and table linen. Its oil is known as linseed oil. In addition to referring to the plant, 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. The plants called "flax" in New Zealand are, by contrast, members of the genus Phormium.

Several other species in the genus Linum are similar in appearance to L. usitatissimum, cultivated flax, including some that have similar blue flowers, and others with white, yellow, or red flowers. Some of these are perennial plants, unlike L. usitatissimum, which is an annual plant.

Cultivated flax plants grow to 1.2 m (4 ft) tall, with slender stems. The leaves are glaucous green, slender lanceolate, 2–4 cm (3⁄4–1+1⁄2 in) long, and 3 mm broad.

The flowers are 15–25 mm in diameter with five petals, which can be coloured white, blue, yellow, and red depending on the species. The fruit is a round, dry capsule 5–9 mm in diameter, containing several glossy brown seeds shaped like apple pips, 4–7 mm long.

The scientific name for flax was coined by Carl Linnaeus in his book Species Plantarum in 1753.

According to Plants of the World Online the flax species has two botanical varieties, Linum usitatissimum var. usitatissimum and Linum usitatissimum var. stenophyllum.