Jillian, which has Latin origins, means "youthful," so it's perfect for reminding baby that they'll always be, well, your baby. This girl's name can also be spelled as Gillian and has the retro-sounding Jill as a nickname. Jillian is the feminine form of Julian or Julius, which means "Jove's child," and is also intrinsically linked to some of the Roman Empire's most powerful rulers.