Here is a flat field in the rain. Does a mass of rock or soil slide down a slope here?

The field has no slope, and nothing slides. So this is not a landslide.

Rain on a flat field: no slope, nothing slides. Not a landslide.

Now consider Elliot's hillside after three days of heavy rain. The soil is soaked and heavy.

Suddenly a whole mass of soil, rock and trees slides down the slope. So this is a landslide.

A mass of soil, rock and trees slides down the slope. A landslide.

Now consider a rocky slope in Alaska on a dry day.

The ground shakes, and a mass of rock slides down the slope, suddenly.

No rain fell, but a mass of rock slid down a slope. So this is a landslide, too.

No rain: the ground shook, and a mass of rock slid down. A landslide, too.

Now consider Elliot's hillside on a different day. A little mud washes down and sits at the bottom.

The hill stays where it is. No mass of soil slid down suddenly, so this is not a landslide.

A little mud at the bottom, the hill still in place. Not a landslide.