If "course" refers to a path or direction:
* Detour: A temporary departure from the intended course.
* Deviation: A departure from the original course or direction.
* Diversion: A change in course, often to avoid something.
If "course" refers to a series of events or actions:
* Backtrack: To go back over previously covered ground.
* Reverse: To go in the opposite direction.
* Undo: To reverse or cancel something that has been done.
If "course" refers to a subject of study:
* Specialty: A particular area of expertise or focus.
* Minor: A subject of study that is less important than the main subject.
* Elective: A course that students can choose to take.
Other possible antonyms:
* Discourse: A formal discussion or debate.
* Procedure: A set of established actions or steps.
It's important to consider the specific meaning of "course" in your context to choose the most appropriate antonym.