system
Given the following post, respond as if you're commenting on Reddit:
human
What is cold fusion?

I have seen the Alcator C-mod at MIT and I know about how they are trying to use plasma to induce nuclear fusion. But the temperatures in their reactor emulate those of the sun, what is this cold fusion business? How does it differ from plasma fusion and how cold is it?
gpt
The original Cold Fusion experiment was performed by Fleischmann and Pons at the University of Utah in 1989.  They published findings in which they claimed that they could use palladium as a nuclear catalyst and have demonstrated energy gain beyond what could be expected from chemical reactions alone.  

It had been known for a while that palladium absorbs hydrogen from the surrounding.  The idea is that, if you immersed palladium in heavy water, it would absorb deuterium ions.  (Deuterium is an isotope of hydrogen and has almost the exact same chemical reacitons.  Through some unexplained method, palladium caused the absorbed deuterium ions to overcome the coulomb barrier and fuse.  Nuclei have the same charge so they are normally repulsed.  The general theory has always been that you need to heat the ions very hot in order for them to overcome this charge repulsion, hence Alcator C-Mod and the tokamaks like it.  Cold fusion suggested another possibility that would not require very high energy plasmas, but could be performed at any temperature.

Unfortunately for everyone, all attempts to reproduce Fleischmann and Pons results failed.  There was even an attempt at MIT in the room that Alcator now sits in.  There was some ugliness involved in the university wanting to patent the results and gain any future advancements, so they refused to show the experimental apparatus to would-be reproducers, forcing people to attempt to figure it out on their own.  To this day, no one knows for sure whether Fleischmann and Pons did stumble on something, or created a faulty experiment.  However, due to the failure of everyone else that has attempted it, the general consensus in the scientific community is that the results were bogus, and cold fusion has never yet been demonstrated.

There are people, such as the Rossi group in Italy, that claim to have made cold fusion and are demonstrating it to media groups around the world.  They have a proposed mechanism for catalysis, but it has never been experimentally verified.  Their experiment has not been vetted by the scientific community.

In summary, cold fusion is a process in which you can cause nuclei to fuse through some catalytic process.  However, it has never been demonstrated in a reproducible way to actually occur.