Tsunami
Tsunami is a huge ocean wave caused by sudden movement of the sea floor.
Think of it this way
Fill a deep baking pan with water. Tap the bottom of the pan hard and fast. The water shoots up in a tall wave across the surface. That is how a tsunami works — a sudden jolt to the sea floor sends a huge wave rolling outward.
Explaining tsunami by grade level
A tsunami is a very big wave in the ocean. It starts when the ground under the sea shakes hard. The wave moves fast across the water. When it hits the shore, it can knock down buildings.