Transistor input and output characteristics
Karan, the bipolar transistor is a two-port device that is not free of return effects. That means: 


While measuring input characteristics (input port) , the conditions at the output port (voltage, current) play a certain role and, therefore, should be kept constant.
Likewise: While measuring output data (at the output port), the conditions at the input port must be kept constant.
In practice, we require to keep all the DC conditions (voltage, current) at the respective ports constant. This is identical to the following requirements (which sometimes are misunderstood): 


(1)  DC voltage constant means: ac voltage zero, short circuit in the small-signal equivalent diagram.

(2) DC current constant means: ac current zero, open circuit in the small-signal equivalent diagram.

I hope this helps.