Gillian \gi(l)-lian\ as a girl's name (also used as boy's name Gillian), is pronounced GILL-ee-an, JILL-ee-an. It is of Latin origin, and the meaning of Gillian is "youthful". Anglicization of Juliana. A common name in the Middle Ages. Actress Gillian Anderson.
For more information, see also the related name Jill.