Allier (UK: /ˈælieɪ/ AL-ee-ay, US: /ælˈjeɪ, ɑːlˈjeɪ/ a(h)l-YAY; French: [alje] ; Occitan: Alèir) is a department in the Auvergne-Rhône-Alpes region that borders Cher to the west, Nièvre to the north, Saône-et-Loire and Loire to the east, Puy-de-Dôme to the south, and Creuse to the south-west. Named after the river Allier, it had a population of 334,872 in 2021. Moulins is the prefecture; Montluçon and Vichy are the subprefectures. Its INSEE and post code is 03.
Before 2018, the inhabitants of the department did not have a demonym. The inhabitants of the department have officially been known in French as Bourbonnais since 2018, a reference to the historic province of Bourbonnais. Until then, the unofficial term Elavérins had been used.