Web Development = Web Design + Web Based Software Development
This is based on my personal experience in the field, not a dictionary definition.
Web Design puts the stress on design, i.e. the visual aspect of creating a website.
Web Based Software Development puts the stress on software, i.e. the underlying application that serves the website.
Web Development is a more generic term that contains both; since it does not stress the aesthetic or technical nature of the field. It entails the development of a website, which both contains the application that serves it (if applicable) and the design of the website that is served (if applicable).