In Calculus, why must the gradient of a specific point of a curve be given by a line that locally touches the curve at one and only one point?

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What I don't understand is why the gradient of a specific point of a curve, the tangent line, must only locally touch the curve at one and only one point. What is the logic behind that? Why does the line that gives the approximate gradient of a point on a curve have to touch the curve at only one point?