What is a rogue/freak wave?

A rogue/freak wave is an ocean surface gravity wave whose wave height is larger than the general expectations. It sometimes severely damages ships or ocean structures. Although the definition is vague and not distinct, the widely used definition is that they are larger than twice the significant wave height. There are many physical mechanisms to generate such extreme waves in the ocean. This chapter includes the introduction, definition, and basic formation mechanisms of rogue waves in the ocean.