Aria

Definition: A solo piece sung. Usually part of a larger work like Opera, Oratorio or Operetta

Definition: Originally an air; a song; a tune; sung by a single voice with or without accompaniment. Now taken to mean a lyric song for solo voice generally having two contrasting parts (I and II), ending with a literal or elaborated repeat of part I. The aria first developed into this form in the early operas; the arias found in an opera, cantata or oratorio usually express intense emotion.

Source: OnMusic Dictionary