An aria (Italian for air; plural: arie or arias in common usage) in music was originally any expressive melody, usually, but not always, performed by a singer. It is now used almost exclusively to describe a self contained piece for one voice usually with orchestral accompaniment. Perhaps the most common context for arias is opera; there are also many arias that form movements of oratorios and cantatas. Composers also wrote "concert arias", not part of any larger work, such as "Ah Perfido" by Beethoven and a number of concert arias by Mozart.
In the 17th century, the aria was written in ternary form (ABA); these arias were known as da capo arias. The aria later "invaded" the opera repertoire with its many sub-species (Aria cantabile, Aria agitata, Aria di bravura, and so on). By the mid-19th century, many operas became a sequence of arias, reducing the space left for recitative, while other operas (for instance those by Wagner) were entirely through-composed, with no section being readily identifiable as a self-contained aria.
ARIA is also an acronym for the Australian Record Industry Association.