To flee means to run away quickly from danger or something frightening. When a rabbit spots a fox, it flees into the bushes. When a fire alarm sounds, people flee the building to get to safety.

The word carries a sense of urgency and fear. You don't flee from something mildly annoying; you flee when you're genuinely scared or threatened. In history books, you might read about families fleeing war zones or people fleeing natural disasters like floods or wildfires. In The Wizard of Oz, Dorothy and her friends flee the Wicked Witch's castle after taking her broomstick.

Flee is stronger than simply “leaving” or “going away.” If you leave a boring party, you're just leaving. If you flee from an angry swarm of bees, you're running as fast as you can to escape. The past tense is fled: “The deer fled when it heard the hikers approaching.”