The 19th century writings of Dumont d'Urville describe an encounter with monstrous waves in the Southern hemisphere. People did not believe the story he told. He was most likely not the first person to survive to tell of this phenomenon which has existed for centuries.
A rogue wave is a wave that is massively larger than what one might expect from the prevailing ocean conditions when it occurs. Rogue waves are characterised by a peak-trough height of over twice the significant height of the sea condition, or by the elevation reached at the peak above the average level of over 1.1. to 1.25 times the peak-trough significant height.
In real life measurement methods are rarely available when a rogue wave appears, and it is thus difficult to confirm its nature; unfortunately, the damage it causes is often the only way to determine its classification.
This damage can be caused not only by the height of the wave, but also by its curve. The speed at which one passes from the trough to the peak differentiates this phenomenon from a wall of water or a gentle hill, however high the latter is.
A distinction is generally made between actual rogue waves and other abnormal waves, which are not caused by the effect of wind on the sea surface: tidal waves (tsunamis), tidal bores, etc.