1. the plane that contains all the lines tangent to a specific point on a surface;
The Collaborative International Dictionary of English v.0.48:
Tangent \Tan"gent\, a. [L. tangens, -entis, p. pr.]
Touching; touching at a single point; specifically (Geom.)
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.
Tangent plane (Geom.), a plane which touches a surface in a
point or line.
Tangent scale (Gun.), a kind of breech sight for a cannon.
Tangent screw (Mach.), an endless screw; a worm.
WordNet (r) 3.0 (2006):
n 1: the plane that contains all the lines tangent to a specific
point on a surface