Cas·tor / ˈkastər/ 1. Greek Mythol. the twin brother of Pollux. See Dioscuri.2. Astron. the second brightest star in the constellation Gemini, close to Pollux. It is a multiple star system, the three components visible in a moderate telescope being close binaries.

castor 2 perforated vessel for sprinkling pepper, sugar, etc. XVII; swivel wheel on legs of furniture XVIII. var. of caster, agent-noun f. CAST + -ER 1. The sp. -or for -er (still current) may have been favoured as being more appropriate to an instrument; cf. razor, mirror, and words in -ATOR.

castor 1 beaver, unctuous substance obtained from the beaver (used as a drug). XIV. — (O)F. or L. castor — Gr. kástōr. The history of the use of castor oil (XVIII) for the pale-yellow oil obtained from the seeds of the plant Ricinus communis is obscure.

cas·tor / ˈkastər/ • n. a reddish-brown oily substance secreted by beavers, used in medicine and perfumes.