A lecture is an oral presentation given with the intention of informing or teaching others about something. Before quoting someone from a lecture, you need permission from the person in question. The permission should be in writing and a copy should be kept for reference.
Lectures are usually not recommended as a source, since the lecture is a secondary source and as the information is not retrievable by others. It is important to remember that even if the lecture is given by a researcher, this does not make the information itself scientific. Before citing a lecture, ask yourself whether the information is not available in a primary source and, if not, whether what you want to cite has scholarly relevance.
It is important to remember that even if the person communicating happen to be a researcher, this does not make the information itself scientific. Since there is no formal quality control on the information given in a lecture, 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.