What is defined as the rapid movement of an object back and forth?

The term that describes the rapid movement of an object back and forth is vibration. Vibration occurs when an object oscillates about a central point, producing a repetitive motion. This can be observed in various contexts, such as musical instruments when they produce sound or in mechanical systems when components move against one another.

Frequency refers to the number of cycles of vibration that occur in a given time period, typically measured in hertz, and while it is related to vibration, it does not define the movement itself. Amplitude describes the maximum displacement from the central position during vibration, indicating how far the object moves from its rest state, but it does not capture the back-and-forth motion characteristic of vibration. Intensity usually refers to the strength or power of a signal, such as sound, not the motion of an object. Thus, vibration is the most accurate term to define this specific action of oscillation.