How to determine if user watched / “completed” the video?

I'm building a course website. It's relatively simple and I need to track overall project progress only by tracking watched videos amount, e.g.

Course 1
5/12 watched

The question is how do you understand that user watched the video?

Some options are: - user opened the video, e.g. clicked ”play” - user watched some % of the video (what would be the correct percent there?) - user watched the whole video

The point here that if user opened the video, watched for e.g. 10 seconds and jumped to next one, would that count as ”completed”?

For some users probably yes, because he explicitly skipped it or found not relevant. Other may return to it though so they might assume it's not “completed” yet.