1. To keep from happening: took steps to prevent the strike.

2. To keep (someone) from doing something; impede: prevented us from winning.

3. Archaic To anticipate or counter in advance.

4. Archaic To come before; precede.

Synonyms: These verbs mean to stop or hinder something from happening, especially by advance planning or action. Prevent implies anticipatory counteraction: "The surest way to prevent war is not to fear it" (John Randolph).
To preclude is to exclude the possibility of an event or action: "a tranquillity which . . . his wife's presence would have precluded" (John Henry Newman).
To avert is to ward off something about to happen: The pilot's quick thinking averted an accident.
Obviate implies that something, such as a difficulty, has been anticipated and disposed of effectively: "the objections . . . having . . . been obviated in the preceding chapter" (Joseph Butler).
Forestall usually suggests anticipatory measures taken to counteract, neutralize, or nullify the effects of something: We installed an alarm system to forestall break-ins.