Oscillation or vibration is the repetitive deviation from a state of equilibrium over time, as well as the transition between two or more states. The changing (oscillating) property can be a coordinate determining the position of an object (e.g., in the case of a pendulum), environmental pressure (e.g., sound pressure), electric voltage (in the case of alternating current), field strength (in the case of electromagnetic waves), brightness (e.g., variable star), etc.
$$T=\frac{t}{N}$$
where,
t— duration of the oscillation;
N— number of complete oscillations.
The period is inversely proportional to the frequency:
$$T=\frac{1}{f}$$
Frequency is the number of repetitive events (usually oscillations, pulses, etc.) within equal time intervals.
$$f=\frac{N}{t}$$
The unit of frequency in the SI system is hertz (Hz): 1 event per second is 1 hertz.