Mega tsunami often hyphenated as mega-tsunami, also known as iminami or "wave of purification" is an informal term used by popular media and popular science to describe a very large tsunami-like wave significantly beyond the size reached by tsunamis. For this reason, there is no scientific definition of a mega tsunami. Informally, the term generally refers to waves beyond the norm for tsunamis, ranging from over 40 meters to giants over 100 meters tall. Note that mega tsunamis often reach higher than their wave height when they meet land, as the water often floods upwards from the force of impact.
Mega tsunamis are related to tsunamis in name only: they do not usually have the same cause or appearance, the only connection being that both are very large scale water movements caused by point events. They are also not the same as freak waves which appear in ocean waters and are often up to 30 meters tall.