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Sound consists of pressure changes which propagate through a medium such as air or water. A sound can result from anything which causes changing pressure waves by forcing molecules alternately closer together and farther apart.
Many sounds exhibit patterns of redundancy in those changing pressure waves. The presence of such a reduncy allows the sound to be analyzed into single cycles. A cycle is the unit of the repeated pattern of air pressure variation. When a sound exhibits such a pattern, one psychological response to that sound is the sensation of "pitch."
In the following figure, a pattern of air pressure disturbances is generated from a source (represented by the red dot in the middle of the figure). The red wavy figure represents the pattern of air pressure changes which constitute the sound.