The word wave has several meanings:

1. A ridge of water that moves across the ocean or other body of water. Waves form when wind pushes against the water's surface, creating those rolling hills of water you see at the beach. Some waves are gentle ripples that lap at the shore, while storm waves can grow very large. Surfers ride ocean waves, and when you jump into a pool, you create waves that spread outward in circles.
2. To move your hand back and forth in the air as a greeting or signal. You might wave goodbye to a friend getting on the school bus, or wave your hand to get someone's attention across a crowded room. The motion resembles the back-and-forth movement of a water wave.
3. A sudden increase in something. A wave of cold weather might sweep across the country in winter. During flu season, a wave of illness spreads through schools. Cities sometimes experience crime waves when problems suddenly spike. The word captures how these phenomena move through populations like water waves move across the ocean.
4. In science, a wave is a pattern of energy moving through space or matter. Sound travels in waves through the air. Light moves in waves (or as particles, depending on how you measure it). Radio stations broadcast on different wavelengths, which determines where you find them on the dial.