The name Jillian originated from the French name Juliana, which itself is derived from the Latin name Iulianus. Iulianus was a Roman family name that is thought to be associated with the gens Iulia, one of the most ancient patrician families in Ancient Rome. This family claimed descent from the mythological hero Iulus, son of Aeneas, who was the son of the goddess Venus in Roman mythology.

The name Juliana first gained popularity in the early days of Christianity due to the veneration of the 4th-century martyr Saint Juliana of Nicomedia. She was a young noblewoman from Caesarea in Cappadocia (modern-day Turkey) who was tortured and executed for her Christian faith during the reign of the Roman Emperor Diocletian. Her feast day is celebrated on February 16th in the Eastern Orthodox Church and on June 19th in the Roman Catholic Church.

One of the earliest recorded uses of the name Jillian can be found in the 12th-century epic poem "The Song of Roland," which tells the story of the heroic death of a Frankish military leader named Roland during the reign of Charlemagne. In the poem, Jillian is mentioned as the name of a character who is the sister of Roland.

Throughout history, several notable women have borne the name Jillian. One of the most famous was Jillian of Thouars (c. 1091-1174), a Countess of Anjou and Duchess of Aquitaine. She was instrumental in securing the succession of her son, William X, as Duke of Aquitaine, and played a crucial role in the expansion of Angevin power during the 12th century.

Another notable Jillian was Jillian Balfour (1654-1705), a Scottish noblewoman and courtier at the court of King Charles II of England. She was known for her beauty and wit, and was a prominent figure in the literary circles of the time.

In the field of literature, Jillian Holtzmann was the name of a character in the 2016 film "Ghostbusters," played by actress Kate McKinnon. Jillian Holtzmann was an eccentric engineer and nuclear physicist who was a member of the team of ghost-hunting scientists.

Jillian Jenkinson (1935-2016) was a British actress and playwright who was best known for her roles in television series such as "The Avengers" and "Doctor Who" in the 1960s and 1970s.

Finally, Jillian Michaels (born 1974) is an American personal trainer, entrepreneur, and television personality. She is best known for her appearances on the reality television shows "The Biggest Loser" and "Losing It with Jillian," where she gained fame for her tough-love approach to fitness training.