no man's land

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

• noun Land under dispute by two opposing parties, especially the field of battle between the lines of two opposing entrenched armies.
• noun An area of uncertainty or ambiguity.
• noun An unclaimed or unowned piece of land.

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

• noun military The ground between trenches where a soldier from either side would be easily targeted.
• noun A place where no one can or should be.
• noun tennis The area between the backcourt and the space close to the net, from which it is difficult to return the ball.
• noun politics, geography Territory, often disputed, that cannot be inhabited because of fear of conflict, especially:
• noun politics, geography Tracts of uninhabited territory close to the Iron Curtain.
• noun politics, geography The stretch of land between the border posts of two contiguous sovereign states, sometimes separated by great distance.
• noun politics, geography Land not claimed by any recognized sovereign state; terra nullius.

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

• noun an unoccupied area between the front lines of opposing armies
• noun land that is unowned and uninhabited (and usually undesirable)
• noun the ambiguous region between two categories or states or conditions (usually containing some features of both)