The term 'prevent from happening' can be expressed in four different ways, each with a different number of letters. AVERT (5 letters) means to turn away or prevent from happening. AVOID (5 letters) means to keep away from or stay clear of something. PRECLUDE (8 letters) means to make impossible or prevent from happening. THWART (6 letters) means to oppose or frustrate the plans of someone. All of these words are used to describe the action of preventing something from happening.