Software that tries to build up a view of the world by asking questions and using the answers to navigate a tree of knowledge. I think...
'Nother way to think about it: a programming style that emphasizes a "knowledge base" and "inference engine" which acts on that base. Forward and backward (inferential) chaining are typical components.
CLIPS is an open-source expert-system development environment in widespread use.
[ Arjen Markus is working on a Tcl-coded ES in 2003.]