Question

define Nuclear fission and nuclear fusion

Answer 1

Splitting of nuclei into two parts is called nuclear fission
Combining of two small nuclei to form single one is called nuclear fusion

Answer 2

in nuclear fusion light neucleas combines into one heavy neucleas and a lot of energy is released in this process, but for initial neuclear fusion process needs ultra high tempatures like at the core of sun which is capable to do neuclear fusion by two H atoms combining to form one hydrogen atom, and giving out enormous radiation
eg-hydrogen bomb works on this principle

now in neuclear fission one heavy neucleas divides to form new light neucleas and the mass diffrence which is caused in the process gives the magnitude of energy release and this process is used in atombombs