The following terms are commonly used in connection with the vibratory motions:
1. Period of vibration or time period: It is the time interval after which the motion is repeated itself. The period of vibration is usually expressed in seconds.
2. Cycle: It is the motion completed during one time period.
3. Frequency: It is the number of cycles described in one second. In S.I. units, the frequency is expressed in Hertz (briefly written in Hz) which is equal to one cycle per second.