A webinar must be a seminar that takes place over the web, right?
It really is exasperating, isn't it.
Sunday rest: webinar. A seminar is a small group of people with a common interest who get together to discuss a topic. Every member of the group is expected to make a contribution.
If this is done via the internet then it is called a web conference.
A webinar also takes place over the internet, but it is a lecture made by one person (or perhaps a few people) to an audience.
The word seminar comes from the Latin seminarium, meaning seed plot. The word web has been around for ages. The Old English form of the word was webb.