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. 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.

Linen is a textile made from the fibers of the flax plant.