In one usage it refers to the new area of land (delta) built up by alluvium from the Mississippi River over the last 5,000 years or so - the "recent delta", in south-central and southeast Louisiana.
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".
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.
As a political region the delta includes the states of Arkansas, Louisiana and Mississippi together with parts of Illinois, Kentucky, Missouri, and Tennessee.