In mathematics, the word ** tangent** has two distinct, but etymologically related meanings: one in geometry, and one in trigonometry.

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In plane geometry, a straight line is **tangent** to a curve, at a some point, if both line and curve pass through said point with the same direction; such a line is the best straight-line approximation to the curve at that point. The curve, at point *P*, has the same slope as a tangent passing through *P*. The slope of a **tangent line** can be approximated by a secant line. It is a mistake to think of tangents as lines which intersect a curve at only one single point. There are tangents which intersect curves at several points (as in the following example), and there are non-tangential lines which intersect curves at only one single point. (Note that in the important case of a circle, however, the tangent line will intersect the curve at only one point.)

In the following diagram, a red line intersects a black curve at their tangent point:

- "And I dare say that this is not only the most useful and general [concept] in geometry, that I know, but even that I ever desire to know." Descartes (1637)

A "formal" definition of the tangent requires calculus. Specifically, suppose a curve is the graph of some function, *y* = *f*(*x*), and we are interested in the point (*x*_{0}, *y*_{0}) where *y*_{0} = *f*(*x*_{0}). The curve has a non-vertical tangent at the point (*x*_{0}, *y*_{0}) if and only if the function is differentiable at *x*_{0}. In this case, the slope of the tangent is given by *f* '(*x*_{0}). The curve has a vertical tangent at (*x*_{0}, *y*_{0}) if and only if the slope approaches plus or minus infinity as one approaches the point from either side.

Above, it was noted that a secant can be used to approximate a tangent; it could be said that the slope of a secant approaches the slope (or direction) of the tangent, as the secants' points of intersection approach each other. Should one also understand the notion of a limit; one might understand how that concept is applicable to those discussed here, via calculus. In essence, calculus was developed (in part) as a means to find the slopes of tangents; this challenge, being known as the **tangent line problem**, is solvable via Newton's difference quotient.

Should one know the slope of a tangent, to some function; then, one can determine an equation for the tangent. For example, an understanding of the power rule will help one determine that the slope of *x*^{3}, at *x* = 2, is 12. Using the point-slope equation, one can write an equation for this tangent: y - 8 = 12(*x* - 2) = 12*x* - 24; or: y = 12x - 16

In trigonometry, the **tangent** is a function (see trigonometric function) defined as:

See also: