The "End", the finale or grand finale. The last lap, last part, final league, last gasp or last breath.
The final part of a play could be called several things: Denouement The finale The epilogue The final act.
A final part can be called a post script, or as an abbreviation, a PS.
"Final" can be both a noun and an adjective. As a noun, "final" refers to the last or concluding part of something. As an adjective, "final" describes something that is last or ultimate in a series.
In Final Fantasy VIII, the spaceship was called Ragnarok.
Dénouement
The home stretch
epilogue
Home stretch
The part of something is called a component. It is a piece or element that makes up the whole of an object or system.
A tenant farmer, otherwise referred to as a share cropper, farms in exchange for part of the final crop.
A small piece or part of something like a piece of glass is called a shard.
The final and shortest answer would be that "the final" refers to the last or concluding part of something, while "the shortest" indicates the least lengthy or briefest version of something. If you need clarification on a specific context, please provide more details!