Tangent

A tangent line curve, or surface; specifically, that portion of the straight line tangent to a curve that is between the point of tangency and a given line, the given line being, for example, the axis of abscissas, or a radius of a circle produced. See Trigonometrical function, under Function.

Touching; touching at a single point

meeting a curve or surface at a point and having at that point the same direction as the curve or surface; -- said of a straight line, curve, or surface; as, a line tangent to a curve; a curve tangent to a surface; tangent surfaces.

A trigonometric function related to the ratio of the opposite to the adjacent side of a right triangle

ratio of the opposite to the adjacent side of a right-angled triangle

a straight line or plane that touches a curve or curved surface at a point but does not intersect it at that point