Term used to describe a searchable database of bugs and issues with a system or piece of software. The normal way for a knowledge base to be created is to take old support issues (i.e. bug or defect requests) and "promote" or archive them into a knowledge base article.
The thinking behind making a knowledge base available to the public is that the customers will search for the solution to their problems before hassling the always overstretched technical support team. However, the knowledge base will often be hidden in the depths of an unusable website, defeating the whole object of putting the thing there in the first place.
A good example of a knowledge base is Microsoft's, at http://support.microsoft.com/ | 
