(Also referred to as mental representation or cognitive representation.)
A representation is a formal system for making explicit certain entities or types of information, together with a specification of how the system does this. And I shall call the result of using a representation to describe a given entity a description of the entity in that representation.
Marr, David, and Herbert Keith Nishihara. “Representation and recognition of the spatial organization of three-dimensional shapes.” Proceedings of the Royal Society of London B: Biological Sciences 200.1140 (1978): 269-294.