They are described as "members of an imaginary race of ugly, aggressive, human-like creatures".
Orc is among 6,000 new words and phrases which have been included in the latest edition of the Oxford English Dictionary.
Slang terms like head-case and minging - famously used by Jade Goody in Big Brother 3 - are also featured for the first time.
The new edition of the dictionary also reflects our growing dependence on computers.
Cut-and-paste, screensavers and search engines are among the words which are now explained by the dictionary.