Fusion is a phenomenon in which two or more entities willingly merge into a singular stronger being. The resulting being has a combination of physical and mental traits from those that formed it and it's mass is the same as the composing entities combined, making it bigger. The multiple minds combine as well forming a single consciousness with it's own unique personality, sometimes even having traits that neither of the components had.
Fusion takes place with the participants performing a dance or pose. After the fusion's effects are over and the participants return to normal, they will be physically drained from the amount of energy the fusion required. It's not clearly known if the duration of the fusion is limited or if it can last indefinitely.