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An option contract that becomes active only when a certain price is reached. For example, one may purchase a knock-in option with a "knock-in price" of $35 and a strike price of $45. If the price of the underlying asset never reaches $35 at any point over the course of the option's life, the option is treated as if it never existed in the first place. If it does, however, it becomes a plain vanilla option (that is, a regular option) with a strike price of $45. See also: Up-and-In Option, Down-and-In Option, Knock-Out Option.