What musical concept involves a rapidly ascending melody outlining an arpeggio, often used as a dramatic opening?

The concept being described is known as a "Rocket theme." This term specifically refers to a musical idea characterized by a rapidly ascending melody that outlines an arpeggio. Such themes are often employed as dramatic openings in compositions, creating a sense of excitement and forward momentum right at the beginning of a piece. The distinctive upward trajectory of the melody captures the listener's attention and sets the tone for the music that follows.

In the context of music, a Rocket theme essentially primes the audience for what is to come, showcasing the composer’s intent to create an energetic and engaging opening. This technique has been utilized by various composers to evoke excitement and anticipation, making it an effective tool in Western classical music.

The other concepts, although significant in their own right, do not embody the specific characteristics of a Rocket theme. Theme and variations refer to the development of a theme through various alterations, while exposition typically relates to the presentation of thematic material in sonata form. A coda serves as a concluding section of a piece, rather than an attention-grabbing opening.