An Etymological Dictionary of Astronomy and Astrophysics

1. A straight line or plane that touches a curve or curved surface at one and only one point. Conversely, a curve or curved surface that touches a straight line, curve, or curved surface at one and only one point.
   

2. In trigonometry, the function of an acute angle of a right triangle defined as the ratio of the length of the opposite side to that of the adjacent side.

Etymology (EN): From L. tangentem (nominative tangens), pr.p. of tangere “to touch,” from PIE base *tag- “to touch, to handle” (cf. L. tactus “touch,” Gk. tetagon “having seized,” O.E. þaccian “stroke, strike gently”); tânžânt, loan from Fr.

Etymology (PE): 1) Sâyân, pr.p. of sâyidan “to touch, to rub,” variants sâbidan, pasâvidan;
Khotanese sauy- “to rub;” Sogdian ps’w- “to touch;” ultimately
Proto-Iranian *sau- “to rub.”

2. Tânžânt, loan from Fr.