What is the term for a propagation of energy that moves back and forth or vibrates at a steady rate?

The term that describes a propagation of energy moving back and forth or vibrating at a steady rate is a wave. Waves are fundamental physical phenomena that can carry energy through various media, such as air, water, or even vacuum, depending on their specific nature.

In the context of physics and, particularly, in areas like sonography, a wave is defined by its characteristics, such as wavelength, frequency, and amplitude. Waves can be described as mechanical, electromagnetic, or matter waves, among others, and they play a crucial role in many diagnostic imaging techniques, where the propagation of sound waves is utilized.

The other terms, while related to wave phenomena, do not accurately describe an ongoing vibration at a steady rate. A pulse refers to a single disturbance or a brief change in pressure or energy, which doesn't capture the continuous nature of a wave. Frequency refers to the rate at which the wave oscillates, while signal refers to a broader concept that encompasses any encoded message or information, not the mechanical properties of energy propagation itself. Therefore, wave is the correct term for the propagation described in the question.