How do you define a ‘rogue wave’ ?

“There’s no clear definition of what a rogue is,” says Paul C. Liu, an oceanographer with the National Oceanographic and Atmospheric Administration in Ann Arbor, Mich.
Scientists don’t have many detailed shipboard measurements of rogue waves because they tend to appear without warning, and bobbing ships make poor observation platforms.
A wave typically achieves rogue status not by growing to a certain minimum size but by exceeding the surrounding waves by a certain proportion. The basis for comparison is an oceanographic parameter called significant wave height, which researchers typically calculate by taking the average of the tallest one-third of the waves in a particular patch of ocean. Many scientists define a wave as a rogue if it’s 2.2 times as tall as the significant wave height. (Science News 11/18/06, p. 328).