Refers to a person who appears on stage or at a podium to give a talk, presentation, or participate in a panel. Common in academic, conference, and formal event contexts.
The presenter at the academic conference presented the latest research findings.
The panelists in the panel discussion exchanged opinions.
講演者 refers to a lecturer or speaker giving a formal lecture. 登壇者 can be used for any stage participant, including panelists or introducers, not only lecturers.
Compound of 登壇 (toudan, 'going up on the stage') + 者 (sha, 'person'). The word is transparently derived from its components.