Diagram meaning : A plan, drawing, sketch or outline to show how something works, or show the relationships between the parts of a whole. A graph or chart. A functor from an index category to another category. The objects and morphisms of the index category need not have any internal substance, but rather merely outline the connective structure of at least some part of the diagram's codomain. If the index category is J and the codomain is C, then the diagram is said to be "of type J in C". To represent or indicate something using a diagram. To schedule the operations of a locomotive or train according to a diagram.