What is the term for a hanging mass that swings back and forth when pulled to one side and released?

The term for a hanging mass that swings back and forth when pulled to one side and released is a pendulum. A pendulum consists of a mass, or bob, that is attached to a fixed point, allowing it to swing freely due to the force of gravity. When displaced from its equilibrium position and released, the pendulum undergoes simple harmonic motion, moving back and forth in a regular manner. This oscillation is characteristic of pendulums, which is a concept commonly observed in clocks and various physical demonstrations.

While other terms in the choices relate to motion in physics, they do not specifically describe the action of a mass swinging in the manner of a pendulum. A spring refers to a coiled device that can compress or extend, but it does not specifically describe the swinging motion of a mass. An oscillator is a broader term that encompasses any system that repeats its motion over time, but it is less specific than "pendulum." A wave relates to the transfer of energy through space, often associated with oscillations of particles but not specifically with a mass swinging back and forth as a pendulum does. Thus, "pendulum" is the most accurate and specific term to describe this scenario.