As an English name, it became popular due to the Fleetwood Mac song Rhiannon (1976), especially in the United Kingdom and Australia.
As an English Christian name, Isaac was occasionally used during the Middle Ages, though it was more common among Jews. It became more widespread after the Protestant Reformation. Famous bearers include the physicist and mathematician Isaac Newton (1643-1727) and the science-fiction writer Isaac Asimov (1920-1992).
The name was popular in France during the Middle Ages (the more common French form is Gisèle). Though it became known in the English-speaking world due to Adolphe Adam's ballet Giselle (1841), it was not regularly used until the 20th century.
There was a medieval English form of this name, Helewis, though it died out after the 13th century. In the 19th century it was revived in the English-speaking world in the form Eloise.
The spelling was later altered to Cordelia when Geoffrey's story was adapted by others, including Edmund Spenser in his poem The Faerie Queene (1590) and Shakespeare in his tragedy King Lear (1606).
In the United States it became more common for girls in the early 1980s, shortly after the debut of the television sitcom The Facts of Life (1979-1988), which featured a character named Blair Warner. The name left the American top 1000 rankings two decades later, but was resurrected by another television character, this time Blair Waldorf from the series Gossip Girl (2007-2012).
A famous bearer was the Cuban-French writer Anaïs Nin (1903-1977), known for her diaries.