Wikipedia Defines Keynote Speaker

A keynote in literature, music, or public speaking establishes the principal underlying theme. In corporate or commercial settings, greater importance is attached to the delivery of a keynote speech or keynote address. The keynote will lay down the framework for the following programme of events or convention agenda; frequently the role of keynote speaker will also include the role of convention moderator. It will also flag up a larger idea — a literary story, an individual musical piece or event.