Notes

Frequency is the number of vibration (Oscillations) completed by the oscillator in one second …

Frequency is the number of vibration (Oscillations) completed by the oscillator in one second. Frequency is denoted by v. Its SI unit is Hertz (symbol Hz). Since v is the number of vibrations per second, therefor, the time taken to complete one vibration is 1/v.
Hence,
T = 1/v or v = 1/T s-1