A "giant tsunami" refers to a very big and powerful ocean wave. It can be caused by events like earthquakes or underwater landslides. When a giant tsunami strikes the coast, it can cause massive destruction and flooding. Full definition
Haiti's quake has also demonstrated that strike - slip faults, which create mostly horizontal motions, can set off landslides and giant tsunamis (usually associated with faults that move in a vertical, thrusting motion). (discovermagazine.com)