In the medieval legend of Tristan and Iseult this was the name of a king of Cornwall. It was also borne by the American author Mark Twain (1835-1910), real name Samuel Clemens, the author of The Adventures of Tom Sawyer and Adventures of Huckleberry Finn. He took his pen name from a call used by riverboat workers on the Mississippi River to indicate a depth of two fathoms. This is also the usual English spelling of the name of the 1st-century BC Roman triumvir Marcus Antonius (Mark Antony).
This is the usual form of the name Elizabeth in Spain and Portugal, though elsewhere it is considered a parallel name, such as in France where it is used alongside Élisabeth. The name was borne by two Spanish ruling queens, including Isabel of Castile, who sponsored the explorations of Christopher Columbus.
In America, this name received boosts in popularity from main characters in the movies The Jackal (1997) and Leap Year (2010).
As an English name it has been in use since the Middle Ages, originally in the form Clare, though the Latinate spelling Clara overtook it in the 19th century and became very popular. It declined through most of the 20th century (being eclipsed by the French form Claire in English-speaking countries), though it has since recovered somewhat.
In the Middle Ages this name was common in England due to the popularity of medieval tales about the Trojan War. It subsequently became rare, but was revived in the 20th century.