'Research Chemical' is a term used to refer to any chemical that has not been well researched and does not have an established long-term safety profile.
'Research chemical' is a term used to indicate a chemical which has not had a history of research or human use, and is therefore considered to be at a 'research stage.' There is no official body which determines a research chemical, or when a research chemical has accrued enough history of use to cease its classification as one. Some have suggested that these chemicals should be called "unresearched chemicals" or another term "Experimental Chemicals". Some are new, while others have been around for years; and little is known about most of them besides first-hand accounts of their use. There are research chemicals of many different types of drugs: while many drugs are qualified as research chemicals, the term itself is more of a flag than a category.
The expression can be considered somewhat of a misnomer, as drugs in this category have often not actually been researched, or at least have almost no history of human use. Many research chemicals in use today were originally discovered and published by Alexander Shulgin in PiHKAL and TiHKAL, and a lot more have been found based on these works. The term partially came from the fact that substances in the recreational markets were drugs that had been discovered in labs and only examined in-vitro or low-level animal studies. However, the 'research' more readily applies to the fact that these drugs were usually found through a process of research - such as through exploring analogues of existing psychoactive substances. Very little to no research has been used to establish the toxicology or human pharmacology of these drugs.
'Legal highs' commonly sold on a grey-area market are usually one or more research chemicals made to mimic the effects of other illicit drugs. There is currently a very large market for the production, sale and use of research chemicals, driven by their implicit legality; vendors and users alike pursue research chemicals to avoid legal troubles encountered from being involved with the more traditional chemicals. Governments tend to ban research chemicals a short while after they become popular, and this, in turn, leads to more being discovered and sold. Some legal systems, such as that of the USA, have moved against research chemicals with acts of law implicitly banning analogues of drugs which are already banned.
As many of these drugs are very new and may not have a well-established safety profile, there are a variety of stronger risk factors in using them. It is likely that many research chemicals have undocumented side-effects, interactions or contraindications.
While there is a certain element of risk involved in all experimentation/use of research chemicals. There are some basic precautions one can take in an effort to make an inherently risky behaviour less dangerous than otherwise.