An eponym is the name of a person, whether real or fictitious, which has (or is thought to have) given rise to the name of a particular place, tribe, era, discovery, or other item. An eponymous person is the person referred to by the eponym. In contemporary English, the term eponymous is often used to mean self-titled, as in "Metallica's eponymous 'black album'". The word eponym is often used for the thing titled. Stigler's law of eponymy suggests that Eponyms are usually false, i.e., things are rarely named after the person who discovered or invented them. An aitiology is a "reverse eponym" in the sense that a legendary character is invented in order to explain a term. Although in actual usage there is some overlap, an eponym may be distinguished from a namesake in that a namesake usually includes a "sake" connection to the source name whereas an eponym name merely is derived from a source name without an additional sake connection.

Political eponyms of time periods

In different cultures, time periods have often been named after the person who ruled during that period.

• One of the first recorded cases of eponymy occurred in the second millennium BC, when the Assyrians named each year after a high official (limmu).

• In ancient Greece, the eponymous archon was the highest magistrate in Athens. Archons of Athens served a term of one year which took the name of that particular archon (e.g., 594 BC was named for Solon).

• In Ancient Rome, one of the two formal ways of indicating a year was to cite the two annual consuls who served in that year. For example, the year we know as 59 BC would have been described as "the consulship of Marcus Calpurnius Bibulus and Gaius Julius Caesar" (although that specific year was known jocularly as "the consulship of Julius and Caesar" because of the insignificance of Caesar's counterpart). Under the empire, the consuls would change as often as every two months, but only the two consuls at the beginning of the year would lend their names to that year.

• Well into the Christian era, many royal households used eponymous dating by regnal years. The Roman Catholic Church, however, eventually used the Anno Domini dating scheme based on the birth of Christ on both the general public and royalty. The regnal year standard is still used with respect to statutes and law reports published in some parts of the United Kingdom and in some Commonwealth countries (England abandoned this practice in 1963): a statute signed into law in Canada between February 6, 1994 and February 5, 1995 would be dated 43 Elizabeth II, for instance.

• Government administrations or political trends often become eponymous with a government leader. Examples include the Nixon Era, Trudeaumania, Jeffersonian economics, Jacksonian democracy, McCarthyism, Thatcherism, Kennedy's Camelot, or Reaganomics.