The name Lilly is a girl's name of English origin. Lilly is ranked #153 on our popularity charts and is often added to lists like English Names with International Style and discussed in our forums with posts like "Rotating Sibling Sets".
From the experts:
Lilly may contain one L too many for some people, though this secondary spelling of a name that's become wildly popular is still a popular choice. And the Lilly spelling does feel a bit less wispy, a bit more like a name as opposed to a mere flower, than the slender and delicate Lily. Lilly suggests the long form Lillian, but it doesn't need to be an abbreviation for anything; Lilly can stand on its own.
However, if you love the flower name but want to make it a little different, you've got to move far beyond Lilly -- or Lillian, Liliane and sisters. You might consider Susannah, which means lily, or a more exotic bloom like Azalea or Lotus.