Jillian is the modern spelling of Gillian, the medieval English feminine form of Julian, which goes back to the Roman family name Julius and is usually explained as "youthful" or "downy-bearded". It gives the friendly short form Jill and was at its most popular in the United States in the 1980s and 1990s. Jillian ranked number 120 in 2000, dropped out of the US top 1000 in 2021 and stands at number 1,266 in 2025.