Jillian is the English form of Gillian — itself derived from Julian / the Latin Iulianus ("youthful, descended from Jupiter"). Jillian Barberie — American TV personality. Jillian Hervey — half of duo Lion Babe. Saint Julian of Norwich is the historical form.
Jillian does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Jillian reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.