Final answer:
A wave is a disturbance that transfers energy through a medium, and can be associated with different types of motion.
Step-by-step explanation:
A wave is a disturbance that transfers energy through a medium. In the context of waves, the word 'wave' can be matched with several options: undulate, billow, move to and fro, and flap. These options describe different types of motion that can be associated with a wave, such as the up-and-down motion of particles, the side-to-side motion, or the to and fro motion. Each of these options may be applicable depending on the specific context of the question or the type of wave being discussed.