- A mass of sediment deposited at a point along a river where there is a decrease in gradient, e.g. from a mountain to a plain. The mass is thickest at its point of origin, and thins rapidly in a downstream direction. In time, adjacent fans may coalesce and extend for many kilometres away from the mountain front.
- A pattern of sedimentary deposit frequently laid down by streams or rivers where they spread out into plains. Alluvial fans from past geological eras are potential reservoir structures.