No unambiguous and widely accepted definition currently exists for the term ‘mega-tsunami’. This is in spite of the rapidly growing popularity of the expression in the scientific literature, especially in recent years following the devastation wrought by the 2004 Indian Ocean and 2011 Tohoku-oki tsunamis. A comprehensive literature search is revealing. We find that although there have been several previous attempts at a definition, the term mega-tsunami has generally been applied in a rather arbitrary fashion to a number of tsunami characteristics, such as wave height or amplitude at both source and distant locations, run-up height, geographical extent and impact. This haphazard situation is undesirable. In response we propose a stricter definition for mega-tsunami that is based solely on initial wave height/amplitude at source exceeding 100 m/50 m respectively. A source-related definition conveniently avoids any difficulties associated with the potential influence of coastal physical attributes (e.g. configuration, bathymetry, geomorphology) on tsunami parameters at affected locations. Using this definition, it becomes apparent that mega-tsunamis can only include those rare events on geological timescales generated by large bolide impacts, violent volcanic activity or oceanic island flank collapse, and possibly extreme tsunamigenic submarine earthquakes. Most seismically-triggered events instead fall into the group of souteigai-tsunamis, i.e. ‘unexpected’ tsunamis, which are considered exceptional according to historical experience and local perspectives.