In some conferences there may be some out-of-schedule guest who has some valuable insights to share with the attendees (e.g. some government official or member of a legislative authority) who is allowed to address the audience with a short speech to present the above-mentioned insights.
What is the proper way to describe that kind of speech in English? Is there a standardized phrase or word for such things? The equivalent in some other languages is something like "intervention" but it doesn't sound correct to me in English.