vanishing point

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

• noun The point at which parallel lines receding from an observer seem to converge.
• noun The point in linear perspective at which all imaginary lines of perspective converge.
• noun The point at which a thing disappears or ceases to exist.

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

• noun The point in a perspective drawing at which parallel lines receding from an observer seem to converge.
• noun The situation in which, place where, or point in time when some object or phenomenon is no longer observable or notable.

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

• noun the point beyond which something disappears or ceases to exist
• noun the appearance of a point on the horizon at which parallel lines converge