Tangent of an arc

Tangent \Tan"gent\, n. [L. tangens, -entis, p. pr. of tangere to
   touch; akin to Gr. ? having seized: cf. F. tangente. Cf.
   {Attain}, {Contaminate}, {Contingent}, {Entire}, {Tact},
   {Taste}, {Tax}, v. t.] (Geom.)
   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}.
   [1913 Webster]

   {Artificial tangent}, or {Logarithmic tangent}, the logarithm
      of the natural tangent of an arc.

   {Natural tangent}, a decimal expressing the length of the
      tangent of an arc, the radius being reckoned unity.

   {Tangent galvanometer} (Elec.), a form of galvanometer having
      a circular coil and a short needle, in which the tangent
      of the angle of deflection of the needle is proportional
      to the strength of the current.

   {Tangent of an angle}, the natural tangent of the arc
      subtending or measuring the angle.

   {Tangent of an arc}, a right line, as ta, touching the arc of
      a circle at one extremity a, and terminated by a line ct,
      passing from the center through the other extremity o.
      [1913 Webster]