The name Ciel is a girl's name of French origin meaning "sky". Ciel and is often added to lists like Unique Girl Names: Unusual and Rare and discussed in our forums with posts like "Names on NB that make you go WOW!".
From the experts:
This simple yet evocative French word name was chosen for her daughter by model Niki Taylor. While Ciel may be pronounced in French with two syllables, many will pronounce it as "seal" and there may be confusion with Celia and sister Ceil.