What is a diagram?

This paper responds to renewed interest in the centuries old question of what is a diagram. Existing status of our understanding of diagrams is seen as unsatisfactory and confusing. This paper responds to this by proposing a framework for understanding diagrams based on symbolic and spatial mapping. The framework deals with some complex problems any useful definition of diagrams has to deal with. These problems are the variety of diagrams, meaningful dynamics of diagramming, handling change in diagrams in a well formed way, and all of this in the context of semantically mixed diagrams. A brief description of the framework is given discussing how it addresses the problems.