These verbs mean to keep from happening or doing. Prevent is the comprehensive term for the group. Used of human agency, it implies precautionary or restraining measures taken to hinder, thwart or block a thing. These measures may involve forcible restraint.

• Armed guards prevented us from entering the palace; They tied him up to prevent his escape.

The measures may be positive steps taken to ward off potential trouble: concessions and reforms made to prevent future riots; negotiations to prevent a stalemate; fabric waterproofed to prevent permeation by rain; it is easier to prevent illness than to cure it. Prevent is used also of a non-human agency or chance cause that acts as a hindrance; in such cases, only the effect of the agent is stressed and no motive is attributed to it.

• Rain prevented us from going (or prevented our going) on the hike; A sprained ankle prevented him from playing in the game.

Stop in this sense is more informal than prevent and has greater immediacy. Where prevent often implies forethought and advance preparation to keep something from occurring, stop focuses completely on responsive action to the circumstance at hand.

• I’m leaving, and don’t’ try to stop me from going.

Further, stop may refer to the halting or ending of an action already in progress.

• Stop him from hitting me; to stop an alarm from ringing; to stop a revolt by the use of armed force.

Prevent , which at first had only the anticipatory meaning, has come to apply also to the stopping of an action at any stage, with the implication that the ultimate consequences of the act have been avoided: The enemy passed the outworks and were barely prevented from capturing the fortress.

To preclude is to prevent by anticipation or to rule out by necessity. This word is used not of persons but of events, circumstances, decisions or the like. It suggests a door shut in advance, as a consequence of a prior action, occurrence, choice or commitment.

• Walls and bars precluded the possibility of escape; Our decision to holiday in New Zealand precludes our going to Fiji this year; Professional duties precluded the doctor from accepting the invitation.

Forestall is like preclude in emphasizing anticipation, but it implies a deliberate deterrent. Whereas preclude points to circumstances that make an occurrence impossible, forestall involves the taking of preventive counter-measures in time to turn something aside or temper its effect.

• He forestalled the anticipated criticism by confessing his faults of his own accord; She forestalled my question by bringing up the subject herself; In 1938, Australia decided to stop sending pig-iron to Japan to forestall increased unrest at home.

Avert , more strongly and more specifically than forestall , implies the wording off of a threat or danger. It points to the taking of effective counter-measures just in time to keep something disastrous from happening.

• He swerved sharply to the left and narrowly averted a collision; Police averted a riot by calming down the demonstrators at the scene of the disturbance.

Obviate is the most formal word in this set. It means to dispose of, referring to a risk, difficulty, objection or the like, which is met squarely and cleared out of the way.

• A settlement out of court would obviate the need for a long-drawn-out lawsuit; travellers cheques to obviate the risk of lost or stolen cash.