Originally: the castor oil plant, Ricinus communis, a shrub native to the Mediterranean, eastern Africa, and parts of Asia and naturalized throughout the tropics, having alternate palmate leaves and seeds which are the source of castor oil and of the toxic substance ricin. In later use also (in form Ricinus): the genus of the family Euphorbiaceae to which this plant belongs.

A tick; especially the dog tick, Ixodes ricinus.