Fusion means the process or result of joining two or more things together to form a single entity. Fusion welding is done by melting the materials of same compositions and same melting points. you can see it in the picture below.
In the above picture, you can see two rods made up of same metal are joined together by melting their joining ends.
The fusion welding is widely used method in construction practices as besides rivets and bolts, there is no other method of joining pieces of metal securely.In case of fusion welding, the metal to be welded is heated up to molten state and resolidification results in completion of the weld. Examples: Arc welding, Gas welding, TIG welding, MIG welding etc.
Fusion welding is used in the manufacture of many everyday items including aeroplanes, cars, and structures. A large community uses both arc and flame contact welding to create artwork.