What is a unification fight?

Question

What is a unification fight?

Answer 1

there are different sanctioning bodies with separate titles the unification fight puts the separate titles together in one fighter so they can claim the be the undisputed champion of the class

Answer 2

A desperate cry for legitimacy in the heavyweight division.

Answer 3

that's when 2 champions fight for being the ultimate champion

Answer 4

Stating it with a little clarity:  The title holder from one of the alphabet soup sanctioning bodies goes up against the title holder from one of the other groups with both titles on the line, so that after this match, one of them will hold both titles.