To frighten means to make someone feel sudden fear or alarm. When something frightens you, it causes a jolt of fear that makes your heart beat faster and your body tense up, ready to react.

A loud crash in the middle of the night might frighten you awake. A scary movie scene might frighten younger viewers. Even harmless things can frighten people: a spider suddenly dropping from the ceiling might frighten someone who dislikes spiders, even though most spiders are completely harmless.

The word suggests a quick, sharp feeling of fear rather than long-lasting worry. You might say a thunderstorm frightened your dog, or that you were frightened when you thought you were lost in a store. Something that causes fear is frightening, like a frightening noise or a frightening dream.

Notice that frighten is different from simply surprising someone. Surprise can be pleasant or neutral, but frighten always involves real fear. When you jump out and yell “Boo!” at your friend, you might startle them, but you've only truly frightened them if they actually felt scared rather than just surprised.