In another usage it is the most recent lobe of the recent delta, the area below New Orleans at the mouth of the river - the "modern delta" or "bird's foot delta".
When natives of the state of Mississippi use the term The Delta they're referring to the west central section of the state, generally between the Mississippi and Yazoo Rivers.
The term can also be used for a larger area where the alluvial valley of the Mississippi is also included. The valley reaches from southern Illinois to the junction of the Red, Atchafalaya and Mississippi rivers in central Louisiana. The alluvium filled a deep valley that had been eroded during the Pleistocene when the sea level was lower.