Formal:
* Lecture: A formal, informative talk, usually given by an expert to an audience.
* Address: A formal speech delivered to a group of people, often on a specific occasion or topic.
* Presentation: A structured and organized talk given to an audience, often with visual aids.
* Monologue: A long speech by one character in a play or other dramatic work.
Informal:
* Talk: A general term for any speech or discussion.
* Speech: A common term for any spoken address to an audience.
* Remark: A brief comment or observation.
Other terms:
* Keynote speech: A major speech delivered at the beginning of a conference or event.
* Keynote address: Similar to a keynote speech, but can be delivered at any point during an event.
* Inaugural address: A speech given by a newly elected official.
* Commencement address: A speech given at a graduation ceremony.
The best term to use depends on the specific context of the speech.