The term 'widening of the mouth' can be described by the word 'DELTA', which is a five-letter word. A DELTA is a landform created by the deposition of sediment that is carried by a river as the flow leaves its mouth and enters the ocean. It is typically triangular in shape, with the apex pointing towards the sea. The widening of the mouth occurs when the river deposits sediment at the mouth, creating a fan-shaped landform. This process is known as DELTA formation.