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.
The Cornish form of this name, Jennifer, has become popular in the English-speaking world.
This name was initially more common among Eastern Christians in the Byzantine Empire, but it flourished in Western Europe after the First Crusade. In England it became extremely popular, typically being the most common male name from the 13th to the 20th century (but sometimes outpaced by William). During the later Middle Ages it was given to approximately a fifth of all English boys. In the United States it was the most common name for boys until 1923.
The name (in various spellings) has been borne by 21 popes and eight Byzantine emperors, as well as rulers of England, France, Sweden, Denmark, Poland, Portugal, Bulgaria, Russia and Hungary. It was also borne by the poet John Milton (1608-1674), philosopher John Locke (1632-1704), American founding father and president John Adams (1735-1826), and poet John Keats (1795-1821). Famous bearers of the 20th century include author John Steinbeck (1902-1968), assassinated American president John F. Kennedy (1917-1963), and musician John Lennon (1940-1980).
The forms Ian (Scottish), Sean (Irish) and Evan (Welsh) have also been frequently used in the English-speaking world, as has the medieval diminutive Jack.