To rappel is to go down a cliff, a steep rock, or a wall on a rope. You wear a harness, you feed the rope through a metal device, and you walk backwards with your feet on the rock, sliding down in a controlled way — not jumping, and not just holding the rope with your hands. Climbers, soldiers, and rescue teams do this. You can use it as a verb (they rappelled down) or as a noun (a long rappel). British climbers often say abseil for the same thing; rappel came from French and is the word you will hear in American climbing and in many films. It is a specialist word: people who climb know it; many other people do not.

垂降、沿繩下降（台灣戶外、登山圈的固定說法）