• In Greek mythology, Castor was one of the twin sons of Leda and the brothers of Helen of Troy and Clytemnestra: see Castor and Polydeuces.
• Castor (α Geminorum) is also a star, named after the character from Greek mythology.
• Castor is sometimes used as a nickname for Drusus, the son of Roman Emperor Tiberius.
• Castor is the genus name of the beaver.
• A castor is a wheel mounted with an offset steering pivot such that the wheel will automatically align itself to the direction in which it is pushed. It is used for example on shopping carts and furniture. See also castor angle.
• The castor bean (a poisonous spurge seed) and its castor oil.
• Castor Oyl is a cartoon character, brother of Olive Oyl.
• The Castor Cracking Group is a demo group from Sweden active on ZX Spectrum.
• Castor is a data binding framework for Java. See castor.org.
• CASTOR is an acronym for Cask for the Storage and Transportation of Radioactive Material
• Betty Castor is an American politician and educator.

The following places have been named Castor:

• Castor, Alberta - a town in East Central Alberta[1], Canada
• Castor - a village near Peterborough in Cambridgeshire, United Kingdom
• Caistor St Edmund , in Norfolk, United Kingdom - Castor being an old name for it
• Castor, Louisiana, United States