The frequency tolerance of a crystal or oscillator defined as the initial deviation of a crystal or oscillator frequency when measured at 25°C. The stability over the defined operating temperature range is defined as the Frequency Deviation compared to the measured frequency at 25°C. Tolerance and stability are typically expressed as parts per million and sometimes as a percentage of the frequency.
Frequency is the rate at which something occurs or is repeated over a particular period of time. The rate of frequency is most often measured in cycles per second or the interchangeable Hertz. 60 cycles per second is equal to 60 Hertz. A thousand Hertz is often referred to as a Kilohertz (kHz).