An online encyclopedia is the one that you get on your computer screen. If you purchase one, the advantage is that it is constantly being updated. Hardbound (vis-a-vis paperback) is the encyclopedia...
The first online encyclopedia was AutoPedia - The Automotive Encyclopedia in 1995. http://www.contentmarketingpedia.com/index.php/about-us/a-culture-of-qfirstsq-g/first-autopedia