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Definition of 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.
 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.
 Touching; touching at a single point
Synonyms:
aside, sunburn, cotangent, diameter, horizontal, obtuse, excursus, divergence, orthogonal, burn, tan, topaz, diagonal, oblique, axis, parenthesis, chord, degree, median, suntan, deviation, arc, excursion, inclined plane, perpendicular, approach, cos, line, minute, outline, divergency, curvature, digression, irrelevancy, divagation, cosine.

touching (part of speech: adjective)
 in contact,
 contiguous,
 tangential
Usage examples:

The tangent sight on the standard of their machine gun is always at 200, and they have not altered the range for three months.
 "Leaves from a Field NoteBook", J. H. Morgan. 
104. The electrons are going of their own accord and the nucleus keeps them from flying off at a tangent, the way mud flies from the wheel of an automobile.
 "Letters of a RadioEngineer to His Son", John Mills. 
I can determine the coefficient of friction from the angle of friction by taking the tangent of the angle.
 "The BirthTime of the World and Other Scientific Essays", J. (John) Joly.