keynote speaker - the main speaker at a conference, seminar, m | Useful English
keynote speaker - the main speaker at a conference, seminar, meeting, etc.
Examples: The keynote speaker at the conference will be the CEO of Apple Corporation. After the keynote speaker finished his address, the panel discussion began.
Video: Ladies and gentlemen... Tonight's keynote speaker needs no introduction from me. Fantastic Beasts and Where to Find Them (2016)
