What is a sudden flash of light in a performance often used as a cue called?

The phenomenon referred to in the question is known as a "bump." In theatrical terminology, a bump specifically describes a sudden, often bright flash of light that serves as a cue for action or a change in the performance. It can be used to signal actors or crew members for various reasons, such as starting a particular scene or indicating a shift in lighting or sound.

While spotlighting refers to a focused beam of light that illuminates a specific performer or area on stage, it is not the same as a bump, which is meant to be a momentary flash for cueing rather than sustained illumination. Similarly, although the term "flash" might seem similar, it lacks the specificity and conventional use associated with a bump in theater terminology. The term "signal" can be applicable in various contexts but does not specifically denote the visual aspect associated with a sudden flash of light used as a cue in theatrical performances.