1.Geometry

A line which touches a circle or ellipse at just one point. Below, the blue line is a tangent to the circle c. Note the radius to the point of tangency is always perpendicular to the tangent line. For more on this see Tangent to a circle.

2. Trigonometry

One of the trigonometry functions. In a right triangle, the tangent of an angle is the opposite side over the adjacent side.