Megatsunami (often hyphenated as mega-tsunami, also known as iminami or “wave of purification”) is an informal term used mostly by popular media and popular scientific societies to describe a very large tsunami wave beyond the typical size reached by most tsunamis (usually around 10 m).^{[citation needed]} A megatsunami is associated with waves beyond the norm for tsunamis, ranging from over 40 metres (131 feet) to giants over 100 metres (328 ft) tall. Note that the waves are often much higher when they meet land, as the water often floods upwards from the force of impact.

Megatsunamis are caused by a very large impact or landslide into a body of water when the water cannot disperse in all directions. For this reason, they are usually a highly localized effect, either occurring when the origin of a tsunami is extremely close to the shore, or in deep, narrow inlets, lakes or other water passages.