thunderthunder1noun (no plural)a loud noise in the sky when there is a storm: There was thunder and lightning.Look at the note at storm.thunderthunderthundersthunderedthundering2verb (thunders, thundering, thundered)1When it thunders, there is a loud noise in the sky during a storm: It thundered all night.2to make a very loud deep noise: The lorries thundered along the road.