The term LENR is often used. The press seem to prefer the term 'Cold Fusion.' Neither is necessarily a correct description of the processes involved, which might not in fact be fusion, and might not even be atomic in origin. It's a technology in which positive results have now been demonstrated beyond any shadow of a doubt, but the underlying science of which is very poorly understood. So, for the want of a better term, the labels 'Cold Fusion' and 'LENR' have stuck.
What we are referring to here is processes taking place under typical chemistry lab 'test tube' conditions which output more energy than the chemical reactions involved are known to release. In some cases, ten or a hundred times more. Most of these reactions take place at the surface boundaries of specific metals. Palladium and nickel are two such. Some experiments are carried out by electrolysis, some by pure heat and pressure. All require quite precise conditions for the effect to manifest itself, a situation which explains why many less-diligent investigators have failed to replicate the original experiments.
Presently the amount of energy extractable per unit fuel, and the feasibility of commercial exploitation, is unknown. What we do know, however, is that we have here a potential game changer -a technology which might have the capability to replace all current forms of power. Including, fossil fuels and existing nuclear. Or, it might only be applicable to a certain range of situations. Too soon to say on this. Of all recent energy innovations this is probably the top of the list for investigation, though. Especially as the cost of doing so is relatively small, and the potential dividend enormous.
The landmark research (although, not the earliest work) in this field was done by Stanley Pons and Martin Fleischmann in 1989. Notably, these were both very distinguished scientists with an impeccable track record. Perhaps inadvisably, their backers decided to publish their somewhat controversial findings of an unexplained source of energy, before any proper review of the data. The result was a catastrophe. Other establishments tried to repeat the experiment and failed. We now know the reason, that they had not been given sufficient information about the conditions required. On the strength of the failed repeats, Pons and Fleischmann were branded charlatans.
The arguments raised for labelling this as 'pathological science' included the observation that no neutron radiation and no transmutations of one element to another were seen, both expected features of a nuclear fusion process. However, Pons and Fleischmann had never declared the process to be nuclear in origin. The term 'Cold fusion' had in fact been invented by the press. Thus, there was in principle no reason to expect such findings.
Its labelling as 'junk science' led to an extreme reluctance by any established scientist to go anywhere near the subject, for fear of ridicule. This situation effectively prevented any further work for twenty years. Only in recent times has the subject been properly revisited, and the findings of Pons and Fleischmann fully vindicated as correct.
Part of the issue with persuading the major research establishments to do work on LENR is the lack of any textbook theory to explain it. Traditionally, energy either comes from chemical reactions or from nuclear processes. There are (allegedly) no other sources, and in all other respects energy cannot be created or destroyed. Thus to claim the existence of a third source of energy is to 'rock the boat' of established science quite seriously.
This lack of underlying theory also makes progress in understanding LENR that much harder. When the Los Alamos scientists first tamed nuclear energy, they already had a fairly good idea which materials to try some experiments with, and what conditions to subject those materials to. This was thanks to the work of Albert Einstein, Otto Hahn, Lise Meitner and others who had already laid-down the theory of how radioactive decay worked, and why it liberated energy. With LENR we have no such pointers to lead us in the general right direction, and thus have to proceed largely at random. If someone can come up with a working theory, then progress will be a lot faster. In particular, we may then be able to predict which elements other than nickel or palladium are likely to exhibit the effect. Which, might lead to the discovery of an even better-performing metal substrate, something which would otherwise take years of random experimentation to find.