A tremor is an involuntary shaking or trembling movement. Your hands might develop a slight tremor when you're nervous before giving a presentation, or your whole body might tremble from cold after jumping into a freezing pool. The shaking happens without you meaning to do it, which is what makes it different from purposely wiggling your fingers or bouncing your leg.

Doctors use the word tremor to describe shaking caused by medical conditions. Some people develop tremors as they get older, making their hands shake when they try to write or hold a cup. Other tremors come from illness, injury, or extreme emotions like fear.

The word also describes shaking in the earth itself. A tremor in the ground is a small earthquake, usually too weak to cause damage but strong enough to feel. Seismologists (scientists who study earthquakes) measure tremors to understand what's happening deep underground. After a major earthquake, smaller tremors called aftershocks often follow for days or weeks.

You might hear someone say their voice had a tremor in it, meaning it quavered or shook with emotion. Whether it's in your hands, your voice, or the ground beneath your feet, a tremor is that telltale shaking you can't quite control.