at the time of sampling the signal,it satisfies the niquist 
criteria(fs>2f,fs=sampling freq&f=signal freq)

It  is  defined as  
the  sampling  frequency  should  be greater than or equal 
to twice the  sampling  frequency  then  we can  generate  
the  original  signal if  the  condition  does not  
satisfy  we  get  the  signal  in  the  distorted manner 
it  is  given  as ( fs >/ 2 fs)

THANK YOU.,
   MEENU PRIYA

@ minu priya :  m sorry but i think you mis-typed it,
instead of the highest freq component u wrote sampling freq
component .. so the real thing is that " whenever a signal
is being sampled, the sampling frequency has to be greater
than the highest frequency component of the message signal
to avoid any loss of information during sampling process" if
this condition is not satisfied, it leads to certain loss of
information. this condition, i.e ( Fs >= 2Fm ) is called the
nyquist rate.

Sampling therom means the sampling frequency must be grater than or equal to Double the signal frequency. this is alsi must be shown as, 
              
                   Fs >= 2f.