Seismic means relating to earthquakes or similar shaking of the earth. When geologists talk about seismic activity, they're measuring how much the ground is trembling and shifting. Scientists use sensitive instruments called seismometers to detect seismic waves, the ripples of energy that spread through the earth when rock breaks or shifts underground.

Understanding seismic activity helps scientists predict where earthquakes might strike and how to build structures that can withstand shaking. Japan, California, and other earthquake-prone regions have strict building codes based on seismic research.

The word has also come to mean any huge, earth-shaking change. When a company announces a seismic shift in how it does business, the change is so massive it affects everything. A scientist who makes a seismic discovery has found something that transforms our understanding completely. If your school announced that recess would now last two hours every day, that would be a seismic change in your daily routine. The word suggests that the impact is so powerful, it's like feeling the ground move beneath your feet.