Some confusion exists in the minds of many geologists regarding the three-dimensional geological setting of a delta and its offshore counterparts which together comprise a dynamic system. In part, this is caused by the lack of a flexible definition and, therefore, understanding of the term. A delta, whether in the present or geologic past, is one segment of a major sediment accumulation, usually occurring on the continental margin. It basically represents the filling of a basin or, in some instances, on a more regional scale, the filling of a geosyncline. In proposing a revised geologic definition, the authors place this sediment mass within its proper framework, recognize the importance of post-glacial and pre-glacial marine geomorphology, and consider the many external forces that affect it. Thus, we recommend that the term delta be redefined as: the sub-aerial and submerged contiguous sediment mass deposited in a body of water (ocean or lake) primarily by the action of a river.