Tormentil (Potentilla), a genus of plants of the natural order Rosaceæ, sub-order Potentilleæ, differing from other species of Potentilla (q.v.) only in the four-parted calyx and corolla. The Common Tormentil (Tormentilla officinalis, or P. tormentilla) is a very common plant in moorish and healthy places in Britain and throughout great part of Europe. It has a large woody root, which has long been officinal, being an agreeable and efficacious astringent, useful in diarrhoea and other complaints, and which contains tannin, gum, and a red colouring matter.