In geometry, a tangent is a straight line that touches a curve or surface at exactly one point, without intersecting it. The point where the tangent line touches the curve or surface is called the point of tangency.

The tangent line is perpendicular to the radius of the curve or surface at the point of tangency, and it represents the instantaneous rate of change (slope) of the curve or surface at that point.

The term «tangent» comes from the Latin word «tangere,» which means «to touch.» In geometry, a tangent line touches a curve or surface at a single point, without crossing it or intersecting it at any other point.

Tangent lines play an important role in many areas of mathematics and physics, including calculus, differential geometry, and mechanics. They are used to study the behavior of curves and surfaces near a particular point, and to calculate rates of change, velocity, and acceleration.

**Here are some additional points to consider regarding tangents in geometry:**

- If the curve or surface is a function graph, then the slope of the tangent line at a given point represents the instantaneous rate of change of the function at that point. This is the derivative of the function at that point, and it can be used to find the slope of the tangent line at any point on the curve.
- In trigonometry, the tangent function is defined as the ratio of the opposite side to the adjacent side of a right triangle. The tangent of an angle is therefore the slope of the line that forms the angle with the x-axis.
- The concept of tangents can be extended to higher-dimensional spaces, such as tangent planes in three-dimensional space or tangent spaces in abstract manifolds.
- If a curve or surface is smooth, meaning it has no sharp corners or discontinuities, then a tangent line can be defined at every point on the curve or surface.
- The tangent line is closely related to the concept of the normal line, which is a line that is perpendicular to the tangent line at the point of tangency. The normal line is often used in physics and engineering to describe forces and motion along curved surfaces.