Question

define time period and frequency give their Si units

Answer 1

Time Period of a wave is defined as the time in which a particle of the medium completes one vibration to and fro about its mean position.
SI unit of time period is seconds.
Frequency  is the number of waves produced per second.
SI unit of frequency is Hertz(Hz)

Answer 2

Time period is the time required to produce one complete wave ( one cycle). Its SI in other is Second.
The number of complete waves or cycles produced in one second is called frequency of the wave. Its SI unit is Hertz.