a violent shaking of the natural world: an earthquake, or a storm that convulses the sea.
The word comes from a verb meaning "to shake," and it names a violent shaking of nature. Almost always that shaking is the ground itself, so the plain English word is "earthquake." Once, in Matthew 8:24, it is the sea heaving in a storm, the same violent commotion turned on the water instead of the land.
In the Bible an earthquake is rarely a neutral piece of geology. It shakes the ground at the moment Jesus dies and again when the tomb is opened, it breaks open a prison in Acts, and it stands in the list of end-times signs. In Revelation a whole series of them marks God's judgment falling and his presence breaking in, building to the greatest quake the world has ever known. The shaking is the sign that God himself is drawing near.
The single sea-storm use in Matthew 8:24 is the same event Mark and Luke describe with a squall word (λαῖλαψ). Where those writers picture a sudden gale, Matthew pictures the sea being shaken, using the very word he elsewhere uses for an earthquake.