Frequency is the rate at which an entity, such as a star, completes cycles of rotation. The frequency of a wave refers to how many times it oscillates in a given time. The number of cycles, or oscillations, per second is usually given the unit Hertz or Hz. Thus light, being wave-like in nature, can have its frequency measured. Many atoms and molecules emit light with well-defined frequencies.
The inverse of frequency, f is the period, P:
f = 1/P
v = f λ