Psyllium is the small, mucilage-coated seed — and especially the thin, pale outer husk stripped from it — of Plantago ovata, a slender annual herb of the plantain family grown mostly in India. Sold as whole seed, flaky husk or fine powder, it is prized as a soluble fibre that swells into a gel in water, used to relieve constipation, bulk the stool and as a binder in gluten-free baking.