Web design is the creation of digital environments that facilitate and encourage human activity; reflect or adapt to individual voices and content; and change gracefully over time while always retaining their identity.
from: A List Apart: Articles: Understanding Web Design

One could just link to A List Apart and be done with it. That would be short shrift to some excellent content though. A List Apart has damped the zealotry a bit in recent years and most of the preachy comes out of empirical observation these days. Take this article and its author for instance. Zeldman used to be all fire and brimstone but now measures his statements (somewhat). Where the article might have lambasted the designer who revels in brochureware, it now quietly mocks.

And underneath it all is a really good article on what web design is and why it isn’t like designing in any other media.