A person or thing that deviates; something that causes deviation from an established path or standard.
From deviate + -or suffix (one who does). The -or suffix creates agent nouns describing someone or something that performs an action.
In geology, 'deviator stress' is a technical term for forces that make materials deform—a deviator is literally the cause of deviation. It's a concept engineers use to predict how rocks, metals, or buildings will break under pressure.
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