An impromptu speech refers to any speech that has to be made with little notice. It is common for impromptu speeches to be given at meetings, gatherings or events by prominent members.
There can be several triggers of an impromptu speech; often unexpected news or a recent event. When people in attendance at events look towards the leaders for a response, an impromptu speech will often be given. The speech is not expected to be eloquent, but will typically contain a core message that addresses a concern or topic of interest. This is in contrast to a planned speech like a wedding toast, which is written far in advance.