Which word best describes an improvisational action or performance?

The term "impromptu" specifically refers to something that is done without prior preparation or planning, making it the most fitting descriptor for an improvisational action or performance. When a performer acts or reacts spontaneously, they are engaging in an impromptu manner, which captures the essence of improvisation—seizing the moment and responding in the moment without a script or a predetermined outcome.

In contrast, "invariable" indicates something that does not change, which does not relate to the fluid and adaptable nature of improvisation. "Ill-conceived" suggests a poorly planned or misguided idea, which is contrary to the creativity and spontaneity that accompany successful improvisational efforts. Lastly, while "innovative" may describe a creative or novel approach, it does not specifically connote the impulsive and unscripted nature of improvisation like "impromptu" does. Therefore, "impromptu" is the most precise term to describe an improvisational action or performance.