Formal:
* Address: A formal speech delivered to an audience.
* Discourse: A formal and lengthy discussion of a topic.
* Talk: A formal presentation or speech.
* Presentation: A formal delivery of information or ideas.
* Exposition: A detailed explanation of a topic.
* Sermon: A religious lecture.
* Harangue: A long, aggressive speech.
Informal:
* Talk: A casual conversation or discussion.
* Speech: A formal or informal address.
* Lesson: A period of instruction.
* Talk-to: To reprimand someone.
* Scolding: A rebuke or reprimand.
The best synonym for "lecture" will depend on the specific context.