It is response of a system when impulse signal is inputed 
to the system.

it is the mathamatical function that describe the outpou 
wave form when the input is exited by unit impulse.

in mathematical term it is response when U(n) is applied ie
value is 1 for n>=0 n zero forn<0

it is response of the system to impulse and which is entirely depend on system parameters

impulse response of the system is response of the system 
when the input is impulse
impulse response is calculated to look at the behavior of 
the system by inserting some other signals which may be a 
shifted version of impulse responses

a dyanmic system responce at a signal that point is called 
impelse responce