A Cycle is a complete vibration or oscillation of a sound wave or an electrical signal.

Think of the movement of a pendulum. A complete cycle is the movement of the pendulum from the center, to one extreme, past the other extreme, and back to the center.

In a sound wave, a cycle is the complete passage of the wave through a point, from maximum air compression, through maximum rarefaction, and back to maximum compression.

The frequency of a sound is measured by the number of cycles that occur in one second.

A wave that completes one cycle per second has a frequency of 1 Hertz (Hz). If it completes 1,000 cycles per second, its frequency is 1,000 Hz or 1 kHz.