noun

• (dated, poetic) An imaginary flower that does not wither.
  "With ſolemn adoration down they [the angels] caſt / Thir Crowns inwove with Amarant and Gold; / Immortal Amarant, a Flour which once / In Paradiſe, faſt by the Tree of Life / Began to bloom, but ſoon for mans offence / To Heav'n remov'd where firſt it grew, there grows, / And flours aloft ſhading the Fount of Life, […]"
• (organic chemistry) A red to purple azo dye used as a biological stain, and in some countries in cosmetics and as a food colouring. See Amaranth (dye).
  "Amaranth, the most common red food colour in both the US and UK, was banned last week in the US after a study suggested that it caused cancer in rats. […] The situation is confused, however, because the test rats were mixed up with the controls in the US tests, and because some tests have indicated hazards while similar tests show the dye to be safe. In April 1975, the joint FAO/WHO Expert Committee on Food Additives (JECFA) gave amaranth temporary approval through 1978."
• (cooking) The seed of these plants, used as a cereal.
  "They [the Aztecs] considered the plant sacred and it played a key role in many rituals. The Catholic Spaniards were particularly disturbed by the practice of mixing a little blood from human sacrifices into amaranth dough, baking it into cakes which were then broken up and eaten by the faithful. […] In the nineteenth century there were reports of rosaries being made out of the stuff, and popped amaranth, mixed with honey, is still used to make a sweet called alegria ('happiness') in Mexico."