tangent plane

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The plane containing all the lines tangent to a specified point on a surface.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun differential geometry Given a point P on a surface M, the tangent plane of M at point P, denoted by TP(M), is the plane passing through P which contains the tangent lines of all the curves on M passing through P.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the plane that contains all the lines tangent to a specific point on a surface