A sound wave is the pattern of disturbance caused by the movement of energy traveling through a medium (such as air, water, or any other liquid or solid matter) as it propagates away from the source of the sound. The source is some object that causes a vibration, such as a ringing telephone, or a person's vocal chords. The vibration disturbs the particles in the surrounding medium; those particles disturb those next to them, and so on. The pattern of the disturbance creates outward movement in a wave pattern, like waves of seawater on the ocean. The wave carries the sound energy through the medium, usually in all directions and less intensely as it moves farther from the source.
The idea that sound moves in waves goes back (at least) to about 240 B.C. The Greek philosopher Chrysippus (c. 240 B.C.), the Roman architect and engineer Vetruvius (c. 25 B.C.), and the Roman philosopher Boethius (A.D. 480-524) each theorized that sound movement might take a wave form.