what is meant by internal energy.................................?

The energy in molecules or atoms or in system is defined due to the random motion collision or anny movment between them.

that energy is called as internal energy.

it depends on many factors such sas degree of freedom and R contsnat and temperatue etc.

@ sharda 
internal energy can be defined as the energy associated with the random , disordeerd motion of molecules . it is separated in scale from the macroscopic ordered energy associated with moving object . 
it refer to the invisible microscopic energy on the atomic and molecular scale. 
HOPE IT CLEARS YOUR DOUBT 
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