cusp, point of—The point of tangency of two curves, the direction of the extension of said curves being of opposite sign; such as one point on the crescent moon or a point on the edge of a convex-concave lens. Can also be applied to the point of tangency of a straight line and a curve where the direction and extension of the line and curve are of opposite sign.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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