What does castor mean?
A castor is, at root, a beaver — the large semi-aquatic rodent of the genus Castor whose dense fur was once so valuable that it drove much of the early North American fur trade. The word survives most prominently today in compounds rather than as a standalone term. Castor oil, a thick, pale vegetable oil pressed from the seeds of the castor-oil plant, has long served as a household purgative and remains widely used in cosmetics, lubricants, and industrial products; notably, the plant bears no relation to the beaver beyond the name. In British English, a castor (often spelled caster) is also the small pivoting wheel fitted to chair legs, trolleys, and hospital beds so they can roll freely, and "castor sugar" denotes finely granulated sugar that dissolves quickly. The word carries a slightly old-fashioned, technical flavour, appearing mainly in trade, medical, and domestic contexts rather than casual conversation, though its compounds remain firmly established in everyday language on both sides of the Atlantic.
A beaver (mammal of the genus Castor) or the fur of a beaver, historically used in hat-making.
"Trappers exchanged castor pelts at the trading post each spring."
This sense is now largely historical or zoological; the plural is "castors".
Castor oil, a pale viscous oil obtained from the seeds of the castor-oil plant, used as a laxative and in industry.
"The pharmacist recommended a dose of castor oil for his complaint."
Almost always used attributively in "castor oil"; rarely stands alone.
A small pivoting wheel attached to the base or leg of furniture or equipment to allow it to roll.
"One broken castor made the office chair wobble badly."
Commonly spelled "caster", especially in American English.
Regular plural, used chiefly for the wheel sense (e.g. furniture castors); the beaver/fur sense takes the same plural but appears mostly in historical texts.
"The new sofa came fitted with lockable castors."
The beaver's musk was once so prized that when chemists found a plant oil resembling it, they simply borrowed the animal's name — and that is how "castor oil" got its name without ever coming near a beaver.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of castor
English "castor" comes from Latin castor meaning "beaver", which itself derives from Greek kastōr; the word is ancient, with cognates traced across Indo-European languages. From the animal's name came terms for its musk secretion, castoreum, long prized in perfumery and medicine. When seventeenth-century European botanists encountered the castor-oil plant, whose oil resembled castoreum in odour, they applied the same name to it, giving English "castor oil" by the mid-eighteenth century. The wheel sense arose separately from the verb "cast" in the eighteenth century, which is why the spellings castor and caster now compete.
How castor is actually used
In everyday modern usage, "castor" most often appears in fixed compounds such as "castor oil" and "castor sugar" rather than standing alone. The bare noun referring to a beaver or its fur is now largely historical or technical (zoology, fur trade). The wheel sense is common but increasingly spelled "caster", particularly in American English, where "caster" dominates both senses.
Easily confused with castor
"Caster" is the standard spelling for one who casts and for a small swivelling wheel on furniture (also "caster sugar" in British English), while "castor" is chiefly reserved for the beaver, its fur, castor oil, and—especially in Britain—the wheel itself.