Final answer:
The term 'Recitative' refers to a style of singing in opera that is designed to advance the plot, featuring speech-like delivery and sparse instrumental accompaniment.
Step-by-step explanation:
The type of singing in opera that primarily functions to advance the plot by using speech-like singing is known as Recitative. Unlike an aria, which is a more melodic and structured piece highlighting a character's emotion or thoughts, a recitative is conversational and moves the story forward, similar to spoken dialogue. Recitatives are less melodious and more rhythmically free, often accompanied by simple chordal instruments like a harpsichord or piano that provide a sparse backing to the vocal line.
A chorus, on the other hand, may include a group of characters in opera, but their main purpose is usually not to advance the storyline in the way a recitative does. An ensemble involves multiple solo singers performing together, and like the chorus, it is not primarily designed to advance the plot in a speech-like manner.