Define delta in geography

A river delta is an extension of a river's mouth, where the faster flowing water from a river, suddenly slows when it reaches an open body of water (e.g. a lake) and so deposits the suspended sediments in a zone around the mouth, causing an elevated river bar. Often you can see these from satellite images (google 'Indus River Delta') and is clearly identifiable as an elevation near the mouth of a river. This raised area is known as the delta.

This is an explanation of a delta (not a definition). It is your job to define it in your own Words however you think best. I hope that helps though :)