What is the difference between fission and fusion? Fission produces large amounts of energy from a very large mass, but fusion produces small amounts of energy from a very small mass.
During fission, a larger nucleis is formed from smaller nuclei. During fusion, a larger nucleus is formed from individual particles.
During fission, a larger nucleus is broken down into smaller nuclei. During fusion, smaller nuclei are joined to create a larger nucleus.

Fission- is the division of a single entity into two or more parts and the regeneration of those parts to separate entities resembling the original. ... The fission may be binary fission, in which a single organism produces two parts, or multiple fission, in which a single entity produces multiple parts.
fusion-is the process by which uninuclear cells combine to form a multinuclear cell. This process is also known as cell fusion. Gene fusion is the formation of a hybrid gene from two separate genes.