Inspite of all the compensation techniques the output voltage is still not zero with zero input voltage. 
Hence a small voltage is applied at the input terminals to make output voltage zero which is called as input offset voltage.

The following are the non-inverting and inverting amplifiers respectively:

Where  V1 =0 gives

Voltage V2 is given by

   V2 = V0 [R1/(R1+ Rf)]

Or  V0 = V2 [ (R1+ Rf)/ R1]

Since, Vios = │V1 – V2│and Vi = 0

Vios = │0 – V2│ =  V2

V0 = [1+(Rf/R1)] Vios

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