Personal communication is direct communication between two or more individuals, for example in the form of a personal interview, an email or a phone call. Before quoting someone from a personal communication, you need permission from the person in question. The permission should be in writing and a copy should be kept for reference.
Personal communication is usually not recommended as a source, as the information is not retrievable by others and there is no quality control. Before citing a personal communication, ask yourself if what you want to cite has scholarly relevance and whether the information is not available in a primary source in a retrievable format.
It is important to remember that even if the person communicating happen to be a researcher or a research institution, this does not make the information itself scientific. Since there is no formal quality control on the information given in a personal communication, you will need to determine for yourself if it is credible and relevant. You can find advice on what to consider when evaluating a source in the section Evaluating informal sources.