Celje is the third-largest city in Slovenia. It is a regional center of the traditional Slovenian region of Styria and the administrative seat of the City Municipality of Celje (Slovene: Mestna obcina Celje). The town of Celje is located below Upper Celje Castle (407 m or 1,335 ft) at the confluence of the Savinja, Hudinja, Ložnica, and Voglajna rivers in the lower Savinja Valley, and at the crossing of the roads connecting Ljubljana, Maribor, Velenje, and the Central Sava Valley. It lies 238 m (781 ft) above mean sea level (MSL).

Celje was known as Celeia during the Roman period. Early attestations of the name during or following Slavic settlement include Cylia in 452, ecclesiae Celejanae in 579, Zellia in 824, in Cilia in 1310, Cilli in 1311, and Celee in 1575. The proto-Slovene name, from which modern Slovene Celje developed, was borrowed from Vulgar Latin Celeae. The name is of pre-Roman origin and its further etymology is unclear. In the local Slovene dialect, Celje is called Cjele or Cele. In German it is called Cilli, and it is known in Italian as Cilli or Celie.